I wanted an internal Z axis for my CNC machine... the parts aren't available in the Openbuilds Store yet, so I cobbled something together. This is a printed part and uses two common "coupling nuts" as the threaded ends for the Z axis. I'm using 1/4-20 because that is what I could easily get at the hardware store.
To use, simply print two of these, hammer in two (1/4-20) couplings nuts, and then put them in each end of a short piece of v-slot. The trick will be getting the nuts spaced apart exactly right. You might need to flip one of the plates around 180 to get the threads to line up well. I did two additional things to "tune" up the feel and reduce backlash. 1. run a tap through the nuts, and run a die over the threaded rod. 2. Cut some aluminum plates so I can adjust the tension between the two coupling nuts. Seems like its going to work great, but this is UNPROVEN at this point, so YRMV. I'm including the OpenSCAD files if you want to make changes.