Do you need a printable file (stl, dxf, 3mf, etc) or dimensions to draw out your own? Would be pretty easy to model up in Fusion 360. However, since plastics handle stress differently than steel, you'd probably have to design in some additional supports (depending on how hard you are on the joystick while playing)
Here is a regular mounting plate:
https://www.thingiverse.com/thing:3446706I know the s-plates have been out of stock everywhere since like before covid. I had to order the last set I got off eBay from the UK.
If you really need a model created, let me know. Maybe I can pull one of mine apart and model it out for you.
Alternatively, you can just route out room for a full regular plate, then make blocks and straps to hold it in place from the back.