Somewhere in the office is a piece of plastic that I cut out for use as the top half of a minimal Mini-ITX computer "case". I have the other half, and both pieces that I've been using for my X terminal. Foo.
Oh, well. I'm not sure I want that exact style for the recording box in any case; I may go back to my original plan and fasten the whole mess (cables, mini-hub, power strip, preamps, and all) to a single lightweight backboard. But it's massively annoying not to be able to find that piece of acrylic.
On the plus side, I have replaced the power supply in the X terminal (freeing up the tiny one for the new box) and rearranged things to make it somewhat more stable.