I tried the keystone spring thing for a while but could never get good contact. Even without powdercoat overspray, aluminum forms a thin layer of nonconductive oxide very quickly and sanding/scraping/brass brushing the inside of the enclosure just leaves a sometimes-conductive mess of shavings.
I ended up with a ring terminal on the footswitch sandwiched under the star washer. I also got some chonky external-toothed star washers. I've added an extra through-hole on my footswitch daughterboard to connect the wire from the ring terminal but of course you can just bodge that wire to the footswitch ground terminal too.
Tightening those suckers down bites though the aluminum oxide layer (and even a little bit of powdercoat overspray) and the pressure keeps the oxide layer from forming.
I especially needed this when switching to the plastic 'innie' style jacks where you screw the nut/sleeve in from outside the enclosure so you can solder the audio jacks right to the main board.