I have made a t-shirt quilt and I used the iron on stabilizer with the bumps on one side and soft on the other. I bought it light weight. It was not hard when finished and it did keep them in place so that the tshirts would not stretch. I can't recall the actual name of the ................oooooooo.......yea fusible interfacing otherwise as I refer to it as "bumpy stuff" anyway you purchase the lightweight and press it to the tshirt and then cut up your square to the size you want.
I was going to suggest maybe the spray adhesive on muslin also. Don't overspray.
I think most of the people I know that have made a tshirt quilt around here feel better with the fusible lightweight interfacing. There is a quilt shop in Albuquerque who from time to time has classes for this you might give them a call. (I know it's far away but maybe they will give you a couple of ideas). The name of the shop is Quilt Works and they can put you in touch with one of their instructors. Good luck on your tshirt quilt.