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.