Custom glass cutting

You could make something out of glass, that would be easy enough, you could silicone it to th elip on the top of the tank and fill the tank up to the spacer.

You would also have to modify whatever light or hood/canopy you have to fit on top of the spacer...
 
hmm. mine would just lay their eggs on the inside of the lid or top, and there's enough humidity to keep the eggs viable. the bigger problem was the snails getting out altogether. make sure they can't escape.
 
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