Originally I was thinking of just a flat piece of stock (metal or plastic) with traditional standoffs for the mount points but thinking more about it, if you made it from bent metal you wouldn't even have to use traditional standoffs...just a tab bent to hold a screw for each mount point.
That's an idea... I never even thought of that. And bending some metal sheets with cutouts is cheaper than building a full enclosure. Perhaps a metal adapter for a premade enclosure. I like your thinking.