I'm looking for an 8mm diameter ball screw with at least 140mm / 5.5" travel, with a compact nut. The channel in the saddle of a 9x20 looks like it can be milled out to just over 3/4" wide without compromising the strength of the dovetail. With a compact nut the sides of the channel wouldn't need to be cut straight down. Could cut through the back end of the saddle to mount an end bearing on the outside.
I've found some sources for new ones like that but my bank account puckered up tight at the $400~600 price! For that I could buy the Thompson kit with ball screws and some motors and drives.