I recall seeing one where the guy had taken some of the
political yardsigns, which are made with a corrugated
plastic. He cut the sections so that it was a force fit on
the sides and bottom, with the sides holding the bottom
one in place.
He said that after he had a good fit, that he had plugged
the exposed openings, but he couldn't remember what he
used, and thought it might have been silicon. Said that
even if water got in, it would still keep colder water from
being able to have much effect on the tank surface. He
seemed to be pleased with the result, because he was
counting on doing it again if he got another commode
and had to replace the tank at the same time.

slushfund wrote in message
...
I found the bottom failed also. I think it's because that's the area that
receives most of the agitation when the tank is flushed.