A little googling found this thread: Tutorial: Fleece Polo Wraps (big pictures sorry!)
Do you have any polo wraps currently? I would think it'd be pretty easy to measure width and length, cut the fleece, fold over the end of the wrap, sew that and sew on the velcro. Of all the things you can make for horses, I would think polos are one of the easier ones. The only reason I've never made my own wraps are because I don't want to buy yards and yards of fleece for only one set. Seems like a great project to go in on with some friends or a pony club or something.