Vintage Simplicity 4538 two-seam top by elegant musings (My note: tie either end of a piece of elastic around middle of upper arms, crossing back. If comfortable to reach forward then that is the right width.
If you have long arms (like me) and a bunch of long-sleeved tees that don't fit after one wash and dry, you're gonna love this idea. DIY – Long sleeves to short. adding long sleeves to a short-sleeved shirt HAHAHA