I did the same thing about 6 months ago with the battery cap from a GGTS... Just drilled the hole where the post inserts the rest of the way
through and tapped the rest of the way (901)
threading then took a 510 to 901 adapter screwed it in the bottom, and adjusted the center pin of the adapter (backed it out) until it made contact.

it screws all the way flush unlike the provari cap.
