Isn't the membrane in the Deck keyboards made from thin rubber moulded in a special shape? I have read it likened to condoms...
I once got a water-proofed mechanical keyboard. It was a cheap keyboard with Aristotle switches (Cherry MX clones) that had been modified to be waterproofed. The metal plate had been replaced with a rubber membrane that sealed against water. There were also rubber plugs over holes in the bottom of the case. No stabilisers.
The openings in the membrane are snug against the keycap's stem. The problem is that the key feel is severely impaired. It was very mushy and the keycaps touch against the rubber when they bottom out creating a suction effect which delays the keys from going up again.
I took it apart and desoldered the switches for something else, but since then I've heard that Aristotle switches don't last very long.
I still have the membrane if you want it, but I have cut away for one or more switches in it because I wanted to try if I could reuse it for switch silencing.