Seamlessly UV Unwrap Unique Shapes

Hello Blender Community! I am fairly new to Blender, however I understand 3D concepts quite well, minus a few specific situations. This is one of those situations I am stumped by. Basically I am creating a modular hill, that uses one corner piece rotated around its origin point to create the hill. In 2D space, the corner piece more resembles a triangular shape with 3 sides rather than the ideal 4 sided plane used in UV Mapping. This causes a limitation, as textures are seamless horizontally and vertically, but never diagonally. When the shape is unwrapped to a UV Mapping, due to the triangular shape, it attempts to map it diagonally rather than vertically or horizontally. I am aware that this is difficult to understand from my explanation, as it was difficult to really explain, so I have attached a photo with a visual representation. Thank you in advance.