The adapter fits very tightly on the axle, this was done on purpose to prevent the wheel from beeing pushed off, when traversing on rough terrain. The M4 grub screw additionally holds the adapter in place (do not overtighten the screw, or else the adapter could be damaged).
The use of a threadlocker (e.g. Loctite medium) for the M8 wheel screw is highly recommend.

• Cut one M4 thread into the holes on the side using a M4 thread cutter. Alternatively you could use a M4 threaded insert.
• A M8x12,7 threaded insert which was installed using a soldering iron, is used. The M4x20 flat head screw connects the adapter and the wheel.

Parts list:
Wheels:
1 set 1/5 RC Wheels, in my case I am using:
Louise RC X-CYCLONE MFT Technology TRAXXAS X-MAXX 1:5

• 2x M4x8 grub screws
• 2x M8x12,7 threaded inserts
  e.g. (https://www.amazon.de/gp/product/B085X28168/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1)
• 2x M8x20 screws

Important: After I had to replace one drive shaft on my mower, I noticed the adapter had a lot of play. Measuring the old and the new shaft diameter, I noticed a difference of around 0,3mm.
Make sure to measure the shaft diameter of your Robomow. I added both the 17,1mm and 16,8mm adapters, feel free to use the fusion360-file to make the changes you want.

I also added a 16,8mm and 17,1mm gauge, so you don't have to print the whole adapter to test the tolerances.