I don't know what medium of material you would intend to use but...
I have experience as a sheet metal worker, "boxes" were my forte, being a ducting guy. But they make some nifty tools that could come in handy if you were using something similar to the thickness of 26guage-28guage sheet metal...say aluminum or something similarly pliable, on a guage chart it shows .0179" which I believe is how you classify metals other than sheet metal(plain old tin). But the tools I refer to would be a barfold, it has grooves that you slip your metal into to make a clean sharp bend. And a button punch, used for fastening two or more sheets together without screws. If you have a local HVAC shop that fabricates their own stuff I might go talk to them. You could possibly give them specs and have them build the rough case for you! Cut out your vents yourself with a pair of snips, and get some louvers t ocover the whole online or at a box store( i.e. Lowes, Home Depot) I could definitely give you tips to tackle it yourself if you would like, just let me know!