>>>... the plastic sheets they use for needle point or something like that. ... to hold the bottom in place if you have sand just stick it in or if you are bare bottom just use some slate on each side to hold in place.<<<
You can purchase the long plastic clips used to hold the spine of report covers together from a stationary store. I thin bead of silicone will hold them in place on the glass. Use three, one on the bottom and two on the sides. You can make the top edge more rigid by "sewing" on a thin wooden dowel. Use a running stich through the top row of holes and over the top of the dowel.