Join Date: Mar 2019
Location: Madison, Alabama
You'd have to fill in the gap below the bottom pane to make up for the loss of the rim, which you already seem to know, but seriously, you're asking for trouble. You could glue in some little glass strips around the inside edges of the tank around the rim to replace the rims, if you really want to go to the trouble, and it might actually work. Solid triangles in the corners.
As for your question, turning the tank upside down and pouring some resin in the gap to fully fill it in should probably help considerably not only to give support to the bottom, but to hold in the sides.
Just using foam, however, won't hold in the sides, so it won't adequately replace the rims.