Molding thin broad pieces; Make sure you create effective sprues & funnels. In two piece molds, without plinths, keep sprues thick enough to well feed the part field, and a funnel with the aprox volume of finished casting, sprue and all.
Vents, vents, vents; At the outer edge of the buck(master), glue lengths of evergreen rod, long enough to exit the mold matrix. to insure resin travel to those extremities.
Glass and what?; Silicone RTV is a compound of silica, and oxegen, don’t ask me how. But I do know degasing with a vacuum tank will disintigrate the molecular bonds, causing lakes* in the mold. When vacuum deairing, let the RTV get all frothy and expand, then colapse no more. It takes only about a minute at 30" of Hg.