Looking awesome.
3D printers can bridge gaps of small enough size, so if your slots are small enough you can orient them face down without supports. Different printers will be able to bridge better or worse, but if you reduce the size of your holes your printer may be able to print it without supports. Another option is to tapper the slot so it isn't flat, but rather it slopes upwards. Here is and image of an object you could use to create the slots with.
(Something like this): 
Hello Porter,
Thanks for the message and for the tutorial.
I think that you mention this information in the tutorial... My apologies, it seems like I have memory issues! I finally printed my tank, It's a small one, 5cm long.
I'm a total beginner in this field, and I got a Creality 3 V3 SE printer 2 weeks ago. Likewise, I used Cura as slicer with the default Creality V3 SE profile (I couldn't find this profile in Prusa). The nozzle is the default nozzle, which is 0. 4mm and I used as PLA the Creality Hyper PLA Filament (1.75mm).
By the way, I was blown away with the easiness of the setup of this printer, and despite the fact that a lot of people say it's garbage, it's working pretty well on my side, the auto bed leveling is also cool.
You can see attached the result of the tank and also the result of the "Benchmark" with Benchy.
I faced some small issues with this first test and model.
For point 1: I'm going to run some tests. I plan to create some cubes and subtract some cylinders in different sizes from them to see what I get as measurement once It's printed, to see how accurate it is.
For point 2 ...
Some thoughts and questions about this experience with Blender:
I feel like modeling hard surfaces as we do in the tutorial, so with Blender is ok if you don't want accurate models, but not really suitable if you need a lot of accuracy. For sure, Booleans don't help, as using the non-manifold feature can lead to unexpected results (like some parts being destroyed or slightly different).
For real functional objects and hard surface objects, would you still recommend Blender, or would you use an app like Tinkercad, Fusion 360 ?
I'll stop here because I have plenty of questions, but that's already a lot!
My tank
Benchy
Sorry for the long response time. I didn't get a notification that you responded.
Anyways, looking awesome!!!
Regarding 1: "I couldn't use the dowels, they weren't fitting. Probably my fault or the parts I printed were too small, and my printer is not enough accurate? I therefore glued everything."
This is not your fault at all. Every printer has its unique margin of error. As I was learning about my printer, I had to model a few different holes and different-sized pegs. It took about 30 minutes of printing but in the end, I found the size that worked for my specific printer. With that being said, some printers do struggle with smaller resolutions and therefore might have some hard limits on what they can and can't do. If you don't mind gluing your pieces, there is nothing wrong with skipping the dowels and just making everything flat so that you can glue them together.
Regarding 2: "For the canons, although the beginning of the print was going well, at some point the printer was like lost. It printed what I would call a 'small bags of knots'. The canons have a diameter of 3.7 mm, It's once again maybe too small for my printer..."
My guess is the size did contribute to the knotting of the printer. Did the knotting start to occur when the cannons became hollow? My guess would be with such a small size, the hollow portion may have been particularly difficult.
Regarding: "Some thoughts and questions about this experience with Blender: I feel like modeling hard surfaces as we do in the tutorial, so with Blender is ok if you don't want accurate models, but not really suitable if you need a lot of accuracy. For sure, Booleans don't help, as using the non-manifold feature can lead to unexpected results (like some parts being destroyed or slightly different). "
I definitely have some strange things happen with booleans within Blender but I am finding strategies that help alleviate these issues. Do you have any screenshots of the unexpected results or destroyed geometry when trying to make it manifold? I would love to try and troubleshoot some of these for you.
Regarding precision, I don't think Blender is going to replace engineering software like SolidWorks anytime soon, but honestly, I have been able to design and print models that were within a fraction of a mm precision and work as intended all within Blender. I think the biggest issue (for me at least) is knowing the limitations of my 3D printer and then modeling within Blender with these constraints in mind. For example, I needed to model a rectangular prism that slotted into a square hole and I found even the orientation I printed the peg changed the margin of error I could print within.
Also, please ask as many questions as you want, I love helping.
Hello Porter,
Thanks for your replies.
No worries, I'm in the same situation. I haven't got any notification here following your reply, there must be an issue...
Regarding 1: "I had to model a few different holes and different-sized pegs" → That's exactly what I did at the end. And by doing this, I figured out that a difference of 0.4 mm was needed to be able to assemble the different parts.
Regarding 2: "Did the knotting start to occur when the cannons became hollow?" → Yes, I think or that could be because of a lack of adhesion. Because after the first little tank, I printed a bigger one, and I still had issue with the canons. That time I'm 100M sure it was because of it, because I saw the canons falling during the printing... I saw I could set up a Brim in Cura, and it helped for the next printing attempt.
Regarding 3: "Do you have any screenshots" → Nope, I don't have any screenshots to share, not anymore. But, when doing the manifold, some vertices were collapsing, merging with others. And that destroyed the general shape. A bit like if you have selected by mistake too many vertices and had applied a merge them all. That's what gave me there was somewhat a lack of control.
"I have been able to design and print models that were within a fraction of a mm precision and work as intended, all within Blender."
→ I'm happy to read it, I prefer to stick with Blender if I can! That puts my mind at ease :D
At least, now, I know that this process is quite common and normal.