You will have to avoid there becoming a tight seal between the key cap and the silicone sheet.
I have got a
whole keyboard that came with a silicone rubber sheet in-between the switches and key caps to act as a splash-guard.
The keys took some extra time on the up-stroke and they made a sucking sound when they did.
At first I thought it was because of the rubber domes in the sheet, but I cut off a dome and discovered that the effect was still there! Apparently, the key cap had formed a seal against the rubber sheet and there was a suction effect!
I noticed later that most, if not all rubber dome keyboards have air channels moulded in them in-between the rubber cups to prevent the suction effect under the sheet.