A loose blast shirt with the back cut will provide a small amount of support, but I'm not sure it will solve your problem. You need to get at the root of why you have these recurrent pec strains. A loose shirt won't necessarily solve the problem. I had a period of time when I kept getting pec strains every 1-2 months. I finally evaluated my technique and training and modified some things and haven't really had any issues for 2+ years now. Here are the "rules" I have made for myself:
1) Don't bench heavy more than a couple weeks in a row. Do moderate weeks or deloads on a regular basis.
2) Don't let your elbows flair out (this wasn't an issue with me but is worth mentioning).
3) Limit or avoid grinding out reps on the BP. Leave a rep or two in the tank.
4) Stretch your pecs and shoulders daily.
5) No pin presses.
6) Train your shoulders so the pecs don't have to take the entire load when benching.
7) Warm-up very thoroughly. I do bar x30-60 reps, 95x12, 135x8, 185x5, 225x3-5, 275x1-3, and then my work sets.
8) Do sets of very light flyes for high reps to stretch the pecs.