Well I have cut all the shirts for one quilt and then ironed on the interfacing today. Good news, I have enough interfacing for all - it came 20" wide and I am cutting 13" squares so using the extra 7" doubled up. I had no trouble cutting the shirts, I put the 13" square down and held it down or knelt on it while cutting, those shirts were not moving at all!
I did have to adjust the iron setting, did the sports shirts on low first, then the cotton Tees on a higher heat.
I wish I had a cutting table but I don't. I just do 2 or 3 shirts, then get up and move around, then go back and do more. Not a problem. I will be so glad when this is over, not my style of quilting at all. But I am glad I can do this for this dad, he is a really nice guy. And his kids will get the quilts this Christmas. He had previously given the shirts to another woman who was going to make the quilts, she had them so long she eventually passed away herself and he had to get them back from her family. Now I am known to procrastinate but I hope I am never that bad.