I put a M8 threaded rod through the two round openings and designed two adapters for it. On the not so often used end a M8 nut is screwed, the adapter is put on (see pictures) and locked with another nut. For optical reasons, an end cap is placed on this nut.
On the other side, the end cap is placed on the threaded rod and a nut is pressed into the provided opening of the star grip. The star grip can be used to pull the entire holder together, thus setting the force required for unrolling. Nevertheless, the roll can still be changed easily.
We have tested this construction for many months on bad gravel roads. It held everything wonderfully.