Hello René
A nice, practical solution to a problem I encountered as well.
I decided to make a wooden box, i am new to TM as well and haven't quite got to grips with adding photos to a reply. I will try and post it separately under the heading "my storage solution"
I made the box low so I could see over it in the rear view mirror, I've since decided it's not essential ( I drive a van with a bulkhead most of the time)
I may make a larger version when I find time. I didn't keep exact costs but it was around £200 for materials, a lot of time was involved however.
The wood is Iroko, I fitted 2 locks, a gas strut and LED brake light.