I wanted a wall mount that keeps the router a little further away from the wall surface, to help with signal reflection and obstruction as much as possible.
I used random #8 wood screws that I had on hand. One screw is likely enough to hold everything, the second screw is really to keep everything upright. Screws should be spaced 35mm vertically.
I printed this in ASA, but any type of filament should be fine. The router fits in nicely, and the two slim protrusions snap into the routers rear recess, holding it just tightly enough. The circular hold is just big enough for RJ45 connectors to pass through one at a time, allowing for 3 ethernet plus the power cable to pass through.

Designed in OnShape here…
https://cad.onshape.com/documents/f35f09346f2f2df7d354f35a/w/aa4e8f0bb06c6ccfdd57711a/e/f2c1904f8f0b24357d8e78d1?renderMode=0&uiState=67cdf076f1a3b56022b454c5