The hubcap turned up; I've hosed it down and I think this should work OK for the lid. I'll probably spray it black.
After browsing eBay for a suitable base, I thought I'd try this 45cm garden saucer which will hopefully be a good fit.
I was wondering about trying some 3mm flexible polyurethane for illuminated pad covers...
...but I think I'm going to use 3mm flexible plywood. (The holes have a narrow 'shelf' round the edge that it can sit on.)
I'm imagining getting this laser cut into pieces that can be neatly glued into the gaps, laser-engraved with central runes which are cut out in an oval shape for the pads. These can have a piezo mounted on the back and be fitted back into the holes with some kind of slightly springy separation.
The 5cm central circle can be fitted with a Bela Trill
Trill Ring is designed to fit within a Neopixels ring, and fits an arcade button in the central hole.
This could be used for things like volume/transposition.
There's a local company which offers various kinds of laser cutting and engraving work on their HPC LS1290...
...but they mainly arrange stuff for weddings (they use it for custom craftwork/design accessories), and they're pretty busy with that at the mo. But they said to call back in October and they'll help with the hubcap.