You might be able to make a knot with a cloth sim in modern Blender, but I think that will be more trouble than it's worth...
With Curves is not recommended, but maybe someone else here knows a trick to make it work...
The one time that I had to make a knot, I tried all sorts of things, but ended up using a few Planes (with Subsurf and Solidify Modifiers) and model and place the visible pieces of the knot (do not try to make the complete knot out of one piece!) where they should go (use reference!). It'll be a combination of rectangulr and more triangular (actually trapezium form) pieces (of course slightly bend). If necessary, you can always do some detail sculpting on the result.
https://cgcookie.com/u/spikeyxxx/projects/christmas-12