Another roof question - best thing to strap items down with?

Vic R's question today about using the camper roof for storage, combined with my planned Easter trip, has made me thing about the folding mountain bikes.

I don't have anywhere else to put them, so I strap them down on the roof.

I have roof rails I welded to the aluminum replacement roof I built - on one side these mount the solar panels, and the other side gives me a walkway or some storage space - it is on this other side I strap the bikes down:



I have been using small ratchet-straps. However, it is very easy to put a lot of tension on these, which could end up causing cracking on the welds of the roof rails, or ending up thinking they are tight, but having them move slightly in-transit and the straps go loose risking coming undone).

I wondered if strong bungee straps would be a better idea, since they would keep tension if stretched rather than go loose.

What does everyone think? Do others have solutions that work well?

Thanks in advance,

Steve.