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You'll need M3 hex nuts and screws long enough to reach the nuts. I had to go with M3x50mm screws. It will depend on the thickness of your door. The holes pass well beyond the nut without interfering with the outside shells, so there is some give on the bolt length. Just make sure you don't go too long and push out through the outer shells.

Place the nut in the bottom, then twist in the handle. Pay attention to the top hole so that it aligns with the bottom. Then place the top nut in the handle and twist on the top, again, paying attention to the location of the hole so that they align.

I printed this on my Core One with 3dFuel PCTG. The gap between the handle the the top and bottom is only 0.1mm. It's a tight fit but with modern, high quality filaments, properly tuned profiles (I use 3DFuel's profile) and modern printers, you should be fine. Might have to whack it with a soft mallet or a piece of wood, but it should settle.

I have included a hole guide for marking the holes for drilling.

Original OnShape document is here: https://cad.onshape.com/documents/5f5925a17869193c18026c88/w/a1139280365911651b8cfb9a/e/92639f1f3db263287602e59e?renderMode=0&uiState=6a46ce6ed3c36be91ac1f3fe