I got an overpriced 250×200mm sheet off Amazon, just because it was easy. It’s definitely available cheaper elsewhere. I was trying to get blank black G10 / FR4 board to match the PCB but it doesn’t seem easily available in small quantities (except from China). I just searched for 1.6mm sheet and this stuff came up (it’s 0.06 inch but close enough). It cuts really well with a hacksaw - didn’t chip or crack at all and cleans up with a file or just a craft knife. I roughed it up the surface with sandpaper to CA glue it together and they’re holding together great.
The key cap set I got only had 1 and 1.5U. There are some 1.25U caps available separately online so I may well do that, thanks!