PE foam in the exploded view suggests that, unlike typical plate foams, switches rests on top of the PE foam. I’m going to try making one myself to see how that changes the sound.
UPDATE: Tried the mod on a hotswap tray-mount with just 1mm PE foam I had lying around and got the board to sound similarly even without a thicker foam on top. The sound reminded me of how soap box sounds when dropped on tile so Wet poppy sound is pretty accurate. I think switches has to be soldered-in for tray-mounts because, with hotswap, switch pins don’t make full contact with Kailh sockets.
My DIY Jelly mod foam for Tsangan:
