Here's what I've got in mind with what I had around me., M8 bolts mounted in plastic electrical conduit, visible from the driver's seat.
View attachment 63440
View attachment 63441
I'd want to use M10 or M12 nylon nuts and bolts for the proper one. The roof slopes towards the front, so the pipe needs to be raised on the bolt at the front.
Idea being that I could either leave it installed all the time, or slot it in place near to the destination with a barrier. It doesn't matter whether I am loaded, unloaded, low or high tyre pressures, whether the barrier is correctly marked or not, if the pipe fits through on that particular day, then the van will fit through. I'd probably put one on each side, in case of a bent barrier, and I think it would be easier to look across to the opposite side of the van to see whether it hits.