What type of mousepad base is best for a glass desk?

Silicone. Foam bases slide on glass because it’s too smooth for friction, but silicone creates a suction effect that sticks like a phone mount to a windshield. You’ll have to peel it up to move it. One caveat: some silicone pads can leave a faint, easily-wiped residue on glass over time, especially in heat or direct sun.