There is a comment on interfacing on t shirts that someone else put up. Your answer will be there. Its within the last 2 pages of postings ie very recent. See edenandandrea's posting

If you find my posting you will also find good answers they just recently helped me with my problems. I ended up using a layer of iron on tear away them another layer of tear away plus I floated a heavy weight cut away and finally got good results. Ruined two t-shirts trying. Good Luck will se if I can post link.