Perhaps an unpopular suggestion, but why not solve the problem by doing away with conference shirts altogether? Is there any evidence that they work as advertising or act as an incentive for people to attend? It just seems awfully wasteful to me - I must have acquired armfuls of them over my career and, apart from a few with particularly nice artwork, most have ended up donated to thrift shops. They always seem to be made of very thin fabric, suggesting they're sourced as cheaply as possible and making me wonder about the ethics of the labor involved in making them.