Ully, to install stuff that "cannot be installed" I always use stuff like this:
However, the sort I use is narrower between the holes and the holes are a bit further apart from eachother. The version I use can be broken between two holes by bending it there a lot of times/cutting it with some serious pliers.
The stuff is flexible enough to shape it the way you want but strong enough to stay firm and tight. If you put this stuff over a regular (cased) convertor you can screw it to the bottom of the cab.
I think this is usually called "spijkerband" here which would translate to "nail-band"....
