We are remodeling the house and one of the things we are adding is a closet for the cat box (happy happy!). I wanted a nice way to let the cats in and out without leaving the door open. There are commercial ones out there, but none of them were able to fit the trim on a panel door, and all of them were either round or touched the floor. I didn't like either method. Thus, lots of quality time modelling!
There are a lot of different models, depending on what you want attached to the parts. To get going, print an inside with the hole trim attached to it. Located it on your door (do a good job centering it) and mark the OUTSIDE of the hole trim. Give yourself some clearance; maybe 1/8" radially. I used a roto-zip with a circle cutter attachment and LOTS of passes from both sides to cut the two arcs. After the cutout drops, trim up the short straight sections.
I highly recommend some quality time spend smoothing the layers so your spouse/SO approves. I found a coarse file worked quite well, but I run a crazy big nozzle so I get strong and fast prints (1.2mm). Expect at least an hour on finishing per side.
Now, the tricky part. You can either print the trim edging separately, or integral to the main part. You only need trim edging on the upper portion; three sections on mine. If I was planning ahead further I would have fiddled the tail so it didn't slop over the center section of the door! Mark the straight edges of the door pockets and cut back your trim everywhere its not needed; file flush. Then, lots of quality time trimming the trim ;-) I recommend a woodworking profile transfer widge (the one with all the pins in a line). You can press it in place tangent to cat hole trim and transfer it to an inch or so of edge trim. A sneaky thing to do it make your trim edging the right height for your door, so at least a little bit of it doesn't need cutting. The trim is single width on my printer, and is easy to clip with flush cutters or a file. YMMV.
I glued the inside and outside to the door one at a time, then added some filler and sanded things to smooth out the joints.
Pick your filament color to match your paint!
Make sure your paint is compatible with your filament!