One of my other hobbies is woodworking. I actually built my own case out of Purpleheart and Walnut, in a form factor similar to a very wide guitar pedal. 9" wide, a little taller than a regular pedal, and probably 4" deep. I’d recommend going for a larger size electronics project box style case, to accommodate the in/out boards and RCA connections. None of the standard, compact type Pi enclosures that I know of would fit them easily.
The 6 RCA in and 8 RCA out boards have ribbon cables to connect to the Octo hat. The ribbon cables are not terribly long, so it might be tricky to get a case that’s just the right size without having the thing in front of you, so you can plan how to accommodate them near the pi. It’s a little unruly. You could probably also just get longer ribbon cables, but the ones it came with just barely worked for what I needed.
The other trouble with a larger case is then you may also need some also USB/ethernet panel jacks to expose some of the ports of the Pi, unless you can situate the pi with the ports you need near the edge of the box, and just have it open for access.
I used this project box successfully with a Teensy 4.0 setup with a cs42448 board (same chip that the Octo uses), 6 RCA ins and 8 RCA outs (without the same baseboard as the Octo has) but the Teensy is much more manageable, size-wise, than a Pi + hat. Eyeballing the box as I type this, it would fit a Pi and a hat with a bit of headroom. You might be able to cram the Pi + Octo + 8 RCA board + 6 RCA board all in to that box, but I’m not sure.
Might be best to just wait for the Pi + Octo to arrive, to get a better feel for what you’ll need to fit in a case, then go from there.