I had a hole in my garage wall after some electrical work. I thought there may be a reason to access this work later, so I didn't have the contractor patch the sheetrock. I made a hatch for it.

The hatch has a frame that is installed behind the sheetrock and a frame in front of it. The frames, when screwed together, grasp the sheetrock, so you do not have to find a stud to screw it into. The holes are sized for #8 wood screws. The length of the screws will be dependent on the thickness of the sheetrock, but 1-1/4" worked for me.

I built many of the parts in a vertical orientation to prevent the hole for the hinge pin from filling up with supports. (see images) The hinge pin is a straight piece of a metal coat hanger.

When you insert the hinge pin:

1. slide it all the way through the door and the front plate, until it is flush with the opposite end of the hinge pin holes of both parts
2. mark the end that is sticking out, so you can trim it to length with wire cutters
3. remove it and trim it.
4. reinsert it until only about 2 or 3 inches (50mm-75mm) extends out of the hinge pin hole
5. slightly bend it (just 5º or so). This give it some resistance to opening & closing so you don't have to add a latch or magnets.
6. push the pin in, until it is fully inside the hole with none extending out.

If you are an Onshape user, you can copy the CAD files and customize the size and remove the lightning bolt or replace it with whatever symbol you want. Here are the file links:

door & lightning bolt - https://cad.onshape.com/documents/ebc3c7d4d9df1da6ee0e2c4b/w/09e3fcd02831635b122908aa/e/f62c147b377076f277e9b57a knob - https://cad.onshape.com/documents/253b560d1eea72ecf6848f98/w/f99d52284f636995d6a1cffe/e/ac958121a61e99a234e43f94 front plate - https://cad.onshape.com/documents/fa153f6b61f2b55b1d645be5/w/c5ee11fcf4b36e80f6b5c05d/e/abf32cbba798e77f90d73bb1 rear plate - https://cad.onshape.com/documents/151776ff351503a6d14f2be5/w/a8a38718fe7410fc6e4cf0f4/e/e881343f65bfe5fc05a27f52 hatch assembly - https://cad.onshape.com/documents/bf963c9bf9a5ffb845942455/w/ef8fa26c0082624112918cdb/e/80022f48375697c88000978f