You could cover the bottom with wax-paper or cling-film(possibly with paper underneath for visibility), and then brush some vaseline on it.
Or, you may get some sturdy plastic that you cut into a suitable size.
I have found some 3mm white plastic-sheet that I think was 150*50cm in a construction store(for roofing under terraces). They might not have the same sheets at your side of the pond though.
Or, you may get some sturdy plastic that you cut into a suitable size.
I have found some 3mm white plastic-sheet that I think was 150*50cm in a construction store(for roofing under terraces). They might not have the same sheets at your side of the pond though.