Protecting the cavity edges before glass blasting?

Hi, I'm in need of some other methods to protect the cavity edges from getting glass blasted, other than the painstakingly slow tape cutting along the edges.
Do you have any other usable and effective ways of protecting the edges?

Thanks!

We had a simple MUD that we needed to blast every 3,000 cycles. We had a mask machined from aluminum to protect the edges during blasting.