I think the roughest part will be getting everything cut to the right size. Not all of the shirts are the same shape or size, so that appears to be the biggest obstacle so far.
Fortunately, I went down to the basement bookshelves and found my "Generation T" book. Inside of it, there's a pattern for a similar project. Their's is for more shirts, but I am confident that I can construct my version using their methods.
(Pictures above from "Generation T: 108 Ways to Transform a T-shirt")
Like, I said before, this project may take a bit longer than my others. However, I'll definitely post the finished result when it's all done. Wish me luck ;)