On further digging, they have the non- name-brand for much less:
High Density Polyethylene (HDPE) Sheeting - US Plastic Corporation
This is basically the same thing, HDPE (High Density Polyethylene) sheeting.
24" x 48" 1/4" thick sheets are currently $27.30 -
High Density Polyethylene (HDPE) Sheeting - US Plastic Corporation
I'm thinking thats a good size to work with and should protect the glass pretty good, and look nice as well, especially as it becomes covered with coralline.

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