If the bottom under the clay is porous, I wouldn't take the chance of disturbing the clay liner. I'd use cover that had a wide base to support it's vertical height.

Something like this:



If you want it taller, they can be stacked 2 high.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).