I did research and watched several YouTube videos. Decided to see if I could make a glass muller myself (unconventionally). The two things of glass I got at the local goodwill store for a $1.20. I tried squeezing the glass tensely (I have a low IQ) to see how sturdy it was and thought my hand was going to break.
Then I got a small funnel from the local dollar store for $2. I have some Master Gesso (1 gallon) that I got from Dick Blick for $33 to prime my own canvases for oil paintings. So I poured the gesso into the small glass container and the put the small funnel on top and used elmers glue to hopefully seal it well. The gesso added more weight to the glass for pressure against the pigment I will be grinding. The bottom of the glass measures 3 3/4 inches wide.
Hope it works.