Actually it is more simple than you can imagine.
Go to a DIY shop and buy a down pipe for a house.
Cut it to length and using straps fix it to the roof carrier.
Usually you don't have to strap the poles in to carry them.
jam the tube from the brush in the door.
We have been running poles like this.
Just at night one has to be careful to remove the poles from the tubes.
We have been carrying poles like this on the top of a Peugeot 405 Estate since April.
I hope this help you.