Personally building your own sounds good. I would make it 3x3x3 just because I'm lazy and wouldn't want to mess up cutting the 2 feet sections. Make sure you don't have too much ventilation with the screen top and make sure it's fine (small) enough so the snake can't get through.
I have a 36inch garter and she's housed in a 46 gallon aquarium. I'm guessing it's 4x2x3 which allows her a lot of room to move around. I think the snake would be happy with being able to stretch out and not touch the sides of the enclosure.
Sorry if this post makes little to no sense, I've worked 34 hours the last 3 days and slept 3 hours last night. I'm a little sleepy
Good luck
Brittany