I mounted it on my dashboard without drilling a hole... I used a hole of a switch of the heatersystem I didn't use anymore, so I mocked up a hole and installed it with no drilling...

But technically you need a hole. there is a piece of alu that comes with the holder and you need to put this on the back of the dashboard (in you trunk) and then you can bolt these two pieces "together" with two bolts...
in front where the hole is there is also a little piece where you can bolt on your tacho so you can turn it the way you want to, that piece is also included but it is used for the moment to mount the tacho under my dash...
Dem