I use kids plasticene. I
I use kids plasticene. I make a worm larger than the hole to be drilled & stick it in a circle to the glass. Then I fill the well with water. Done it that way for years. I now use diamond holes saws up to 2″ rather than a circle cutter. Tip- pressing the plasticene on the glass dry & it when your srilling it leaks just press it until the leak stops.
Aslo rememebr the water in the well spills as soon as the hole is thru so I drill on a terry toweling rag.