As I had the switch apart I thought I would experiment with some foam that I had lying around - about 6mm thick
So I popped in a couple of pieces in each side of the springs to keep the pressure as even as possible.
After putting everything back together the result is pretty good. The button is now firm but momentary - I can still get it to latch if I push hard either way but we will see how it goes when I get the STS module next week. I reckon it might 'give' over time and need some stiffer foam but it's only a 10-minute job to replace it so not a problem.