What ever you do don't use bondo or fiberglass. Bondo is porous and will absorb water, swell and crack. Fiberglass won't adhere to a high fired pot...especially with constant moisture present. It too absorbs water, unless you get into applying gelcoat to seal off the raw glass. If you're planning on filling tape off one side of the crack so that you have something support your filler. Aluminum tape would work well. Fill the crack with something along the lines of marine epoxy. Don't expect the pot or repair to last more than a year or two.