Got a pair of snips, a rotozip/dremel, and maybe a metal break?
open your phone book. Look up hvac. Start calling. Ask if they use "pre-fab" duct or if they make their own. If they give you static, be honest and tell them you have a project for school (if you look young enough) that needs some sheet metal work. Don't try to explain what you want on the phone. They wont get it. Instead, offer to show them what you want and take the whole thing up there... I think you got it from here.
If they cant/wont cut out for the ports, no problem if you got that dremel/rotozip like every man should have right?
Money: the whole panel with a lip and cut-outs no more than $50.00
just the panel with a lip no more than $35.00
Or if you had the tools you could make it yourself but then again if you had those tools and knew how to use them you more than likely wouldnt have bought a case at all... Just my worthless hat thrown in.