I've got a couple 256-button monomes (no longer in production, basically -- they only make smaller models now) and waited twelve years for something velocity sensitive with as many buttons to come around. Eventually the Lumatone did, and now the monomes sit in a closet -- but I'd really like something with smaller buttons, because I divide the octave into 58 pieces, which means I can barely reach an octave on the Lumatone (whereas I could easily reach across three octaves on the monome).
If you're familiar with hardware design, it wouldn't be terribly hard to make a big grid of pressure-sensitive buttons using force-sensitive resistor material (Sensitronics makes some that they sell approximately by the foot, possibly velostat or a similar material could work too) laid over the top of pairs of interdigitized fingers of copper, connected through a bunch of multiplexers or similar to the ADC inputs on a microcontroller. The buttons can be whatever semi-squishy material you want to lay over the top of the pressure sensors.
(I bring this up because I'm working on a similar project, but not with a regular grid. My keyboard has 113 pressure-sensitive keys. I've been using JLCPCB, who are surprisingly cheap even for large boards and even when buying in quantities of 5 or so.)
I've been looking into this (thanks for the pointer to Sensitronics!), do you have any pointers to a manufacturer of squishy material pads? (silicone I guess)
Sensitronics was recommended to me by Roger Linn one time I met him when he happened to be in town for an event. They apparently make the sensor in the Linnstrument.
What I do is I glue a sheet of 1/8th inch birch ply to a sheet of felt using liquid hide glue, then use a laser cutter to cut that into keys. (They're irregular polygons, but you could do the same easily with squares.) I have another piece of laser cut plywood (or the "negative space" of the same plywood, but with the felt removed) that serves as a frame to hold the keys in place. I use packing tape on the back of the frame, sticky side up, to grab onto the felt so they don't all fall out when you turn it upside-down. The felt gives the keys a satisfying squishy feel. Wood screws hold it all together.
In other words, from top to bottom a key is comprised of 1/8th inch birch ply, a layer of glue, a layer of felt, a layer of tape sticky side up, a layer of FSR film, then ENIG-coated copper traces on an FR4 PCB with the solder mask excluded. I also have another layer of plywood (1/4") on the bottom for structure.
I made an early attempt at using some kind of silicone rubber, but I wasn't successful. It might be doable and I imagine there are companies that can make that stuff for you but in the end I decided I liked wooden keys better and they were just plain easier.
I don't have leds on the keys, but if you want that, WS2812's are surprisingly cheap and could be placed by a pick-and-place machine by the hundreds -- if you can figure out a way to get the light to shine through all the intervening opaque parts.
I considered it! But I'm a piano player, and I like to be free to keep my hands wide apart, which I think requires more tactile feedback than the Linnstrument provides.
I've got a couple 256-button monomes (no longer in production, basically -- they only make smaller models now) and waited twelve years for something velocity sensitive with as many buttons to come around. Eventually the Lumatone did, and now the monomes sit in a closet -- but I'd really like something with smaller buttons, because I divide the octave into 58 pieces, which means I can barely reach an octave on the Lumatone (whereas I could easily reach across three octaves on the monome).