Has anyone tried a cork sheet? It is pretty much water proof, a little spongy and thin.
We are using these various mats are a gasket material so almost all traditional gasket materials will work.
Currently, I'm using a yoga mat, the black foam used under floating laminate flooring, and a pc of top end hypo-alergenic carpet pad under glass & acrylic tanks.
Shock absorption has some value, but the leveling effect IMHO is much more important. When I do a 50% W/C I hear the load being taken off the stand by the creaking of the plywood top my 40 gl sits on. Being an Acrylic it will be more forgiving flex then the joints on a glass tank will be.