https://www.ebay.com/itm/TEMCo-Heavy-Duty-Cast-Aluminum-Foot...
I'm not sure how you'd connect this as an ancillary USB keyboard. Seems like you'd need an Ardunio or something?
I'd love to have a few of these to control DAW apps where I don't need the hassle, cost and complexity of Midi
If you have only one or two switches, you can use the DIRECT_PINS configuration to directly put one switch per input pin rather than the more conventional switch matrix approach. You can use the XD002 code as a starting point (https://github.com/qmk/qmk_firmware/tree/master/keyboards/xd..., https://github.com/qmk/qmk_firmware/blob/master/keyboards/xd...). It uses a DigiSpark rather than the more common Arduino Pro Micro. The DigiSpark is a bit fiddly and has very little program memory, but it is very cheap and simple.