If you're in the mood to DIY...
Get some wall molding from a place like Lowe's or Builder's Square. (i don't think Home Depot sells it, at least i know they don't sell lumber). They should be able to cut it to the length you need, and cut the ends at 45 degree angles. They should also have the plexiglass and the little clips to hold everythin in place. They're usually in the aisle with the stuff like nails and screws. You may need to go to an art supply store and get a heavy matte board for the backing. Paint and/or finish the molding as desired.
Definitely more effort, but cheaper in the long run. Don't forget, measure twice, cut once.