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.
It's not clear from their website, but if you order one of these evaluation boards, it comes with a bunch of small swatches of FSR material: https://www.sensitronics.com/products-xactresponse.php
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.