[ranxerox]

I bought commercial ones made out of steel, They will last longer then the van does.
The half inch ones come close to fitting the holes in the roof.
I have used them at work, The rubber inserts inside them will last twenty years or longer.

They look something like this except they are made out of steel, There is a nut one one end you can tighten down to hold it in the roof sheet metal.

 

[ranxerox]

Sunlight is hard on certain types of wire insulation.