This probably last of this pond series, I've completed waterproofing those access panel using heavy duty plastic coated canvas and sealant.
Panels are sealed with Silastic sealant (Acetoxy Silicone Sealant) then canvas is stick to pond floor covering the panel with a layer of Silastic, spreading it outward to form a seal.
How I wish I have aviation grade PRC 1432 sealant.
Finally 3 layers of pond seal around the canvas. The photo was taken at night, so a bit fuzzy.
Tomorrow is "D" day, water in... :)