I use a reverse bowden tube on my MK3 but I like to use two of the PC4-M6 fittings. I had printed a z-top that had a built in spot for those but in anticipation of hooking up my MMU2, I decided to print a version of the z-top by jltx that does not contain the extra bits.
However, between now and actually getting my MMU2 hooked up, I still needed somewhere to connect a reverse bowden and I didn't want to attach anything to the frame so I created this thing that would just plug into the hex hole on the z-tops of Prusa printers. The images show the orange stock MK3 z-top and a black version of this thing by jltx: https://www.thingiverse.com/thing:2779123
Depending on tolerances, you may need to adjust the scale of your print slightly to get a good snug fit. I didn't feel like fiddling so I put a single layer of blue tape on it and it works fine. The threads for the fittings are modeled in so there is no tapping required.
Use 4 perimeters. I printed at .2 mm in PLA and the threads worked well so you should be able to print from any material in any resolution you prefer and it should still work.