Not just the gap, also the depth of the ‘ledge’. If it is thicker than the rail then it won’t tighten up properly. A case of try it and see. I haven’t tried those.Thanks. I actually just spoke to a helpful chap at camper warehouse who told me these are same as many rails used in ambulances and aircraft etc so advised looking on Amazon. So with that in mind, can someone confirm that something like this would work please? It has dimensions on the pics. I think the key is the gap between the holes on the rails.