This is a an adapter for filament pass-through from a filament spool holder outside of a Lack Enclosure, to a reverse-bowden tube, used to prevent filament unspooling and sudden filament tension changes on a direct-drive extruder.
I've been using this for months with my Prusa Mk3 and my (extended) Lack enclosure (with my enclosure parts, which I'll be sharing soon, which are improvements on the Prusa design)
Update: 01/25 - I've uploaded version 2 of this which has an M6x1 thread for coupling the two parts together, rather than relying on friction fit into the enclosure. The parts will need to be lined-up and then threaded together when inserted. To print the threaded long tube I'd definitely recommend using a brim for stability, and to print it SLOW (and use a max layer height of 0.2mm on both parts, lower is better). I've left the unthreaded parts available for those that might struggle to print the threads.
The inside part (with the thread) should be printed at no greater than 0.2mm layer height.
Just screw a PC4-M6 tube adapter into the inside part, and then drill a hole all the way through the top surface of the enclosure, and insert the top (long piece) and bottom (threaded part) until they fit together (a friction fit into the table should suffice, but you can glue if desired).
For the main hole drill using a 1/4-in or 6.5mm drill bit from the top of the enclosure downwards (suggest ensuring that the printer is not in the enclosure, so as to avoid dust and debris causing issues).
Then once that hole is all the way through, open up the hole on the inside of the enclosure (the enclosure top is hollow) to 8mm or 5/16-in
You can then attach a PTFE tube into the adapter, and onto the reverse-bowden adapter for your printer (I use the Skelestruder extruder on my Mk3 printer, and made my own adapter, but Skelestruder now has an official adapter). Make sure that the tube is just long enough for the lowest side positions that the extruder can move to.
You can then just feed the filament from the top of the enclosure, and it should feed straight into the reverse-bowden tube.
Just remember to disconnect the reverse-bowden tube when unloading filament, so you can snip the blobby end off the filament so that it doesn't get stuck in the tube.