i had the same question, i suggest picking up some acrylic and doing it ur self, plus if u mess up, u can always make another!
use a dremel with a cut-off wheel to take off the excess of the screws9if ya dont have a dremel, i suggest u invest in one, it really makes any job MUCH easier!), use 90' brackets along the sides and back, drill holes for the thumbscrews in the back, and i think u could ad a nice little customization like that, just have a few brackets every 2 inches or so, what ever u want, or just one that runs all the way back, and if u want to just have a spot for the thumbscrews in the back so that u dont have to cut off as much use one screw for every small bracket.
make sure the brackets are flush, and if u want to use something else for the lip, try to use what jaxdolan was talking about, or pick up some superglue and be creative!
its ur rig/box/computer! (what ever u wanna call it.) and heck, it sure is gonna be under 50 bucks!always use saftey glasses!
guess thats it,
good luck with the mod, for painting i suggest letting ur paint sit in warm water before painting, and im sure u know all the priming sanding and painting steps so i wont bore u with that!