Trimming a hole in drywall

I want to cut an "L" shaped hole in the 1/2" drywall behind a flat-panel TV to allow access to all the connectors from the other side of the wall without taking down the TV (a two-man job). Does anyone have any ideas how to trim the hole to make it look nice and not shed powdered drywall as the cables pass through the hole? Is there something specially made for this? I thought of slitting a hose and running it around the edge, just folding drywall tape around the edge and mudding it in, or making a wooden channel using picture frame stock, etc., but there must be a better way.

You can buy an 8' stick of U-shaped plastic designed to cap the edge of drywall. Check out the section at the big-box store where they sell corner bead and other edge treatments. It's cheap. All that I've seen are white.

Hmm. Somehow I missed that; I'll look. White works great. Thanks...