Definitely on the right track, will be very nice momento!
All you need are 'coping' strips - think of these as frames for the squares you have for the t-shirts - for the big t-shirts, you may not need any; for the smaller ones, add a strip of fabric to the top and bottom and/or sides, to make it the same size as your bigger blocks.
Your target, likely, is to make all the blocks the same size - by adding strips to the smaller blocks. You could do the strips just boxing in the t-shirt, or log-cabin style, or attic-window style.
Hope this helps!
Linda