I also put two O-rings on each pipe to prevent sliding while transporting the tanks in my car. I recently removed the rings as I got a rubber matt for my trunk.
Here are the dimensions that I used and should fit the majority of tanks. The pipes are 16in long and each robe is 4ft. For two tank holders a’ two tanks you will need:
8 foot 2in PVC pipe (6 x 16in)
16 foot nylon robe (4 x 4ft)
8 plastic ties
All over I spend about $24 for all the material including the O-rings. I think it's a fair price for four tanks and the thing takes almost no space if it's not needed. I am using it now for over 6 years and yah, some SCUBA tanks got throwing on it ...