Want to use my phone or a small tablet for navigation. Can't find a decent holder which can stand a little offroad abuse...
Don't want anything on top of the dashboard (problems with Belgium MOT!).
Don't want to drill to many holes in my dash.
When driving offroad, phone/tablet should stay in place and doesn't vibrate to much.

Any suggestions on buy's, or examples for me so I can build something bomb proof?

That restriction's a bit of a challenge.
You could try a vent mount - if you removed the vent and modified the fingers with cable ties or screws etc to prevent it popping out or:
RAM mounts (forget the suction pad - note how it's attached to the rest of the arm)

The off the wall idea would be a sunvisor attachment...

That sunvisor idea might be not that bad...

Last 2 trips to the UK, I have forgone Garmin and gone with Google maps on an 8" tablet as the app now has a download map function. Tablet is a Huawei MediaPad T3 8" Tablet - (Qualcomm Quad-core 1.4GHz, RAM 2GB, ROM 16GB, IPS-Display) - Grey (about £100)
I also have netflix and amazon prime subscriptions so I downloaded some video's onto the storage card for waiting at the ferry etc.