It is a fairly simple thing to make. Hubby tweaked the design a few times, but now Lil'c barely uses O2 and we don't use it any more, which is why it is inside. We have used it in both a hatchback and a van.
PVC pipe (slightly smaller than the tanks so it will grip)
3/4" plywood (measure from the floor to the top of the back of the backseat x width of the back of the backseat)
Extra strip of 3/4" plywood 4"x needed length (only under small bottles)
Tie-down straps (auto supplies)
Cut large plywood to size. Notice the notches in the bottom to allow the tie-down straps to wrap around the back of the seat and hold it in place.
Cut PVC pipe to length and then make ONE additional cut down the length which will allow the pipe to expand and grip the tanks.
Screw PVC to the board making sure the vertical cut is facing out.
Screw the strip of plywood under the PVC for the smaller tanks. This will give them something to rest on, otherwise they will slide right through. The large tanks will simply sit on the floor.
Spray paint for appearance. We went with black since we had it on hand, or you could try and match your vehicle's interior.
Fairly simple process :).
This allowed us to make use of our full trunk space while easily being able to trade out tanks. You could easily tell the empty from the full by the white caps. We kept the small tank in use in the transport bag with its valve on and a valve on whichever large tank was currently in use. This allowed us to carry 8 small tanks (notice only 7 slots) and 2 large tanks wherever we needed to go, including 2 4-day trips to see family 14 hours away! On those trips we also brought along the concentrator and used it as much as possible to save our tanks.
Although I will tell you, driving around with that many tanks in the back can make you nervous of other people's driving, but it is certainly better than them rolling around if you are carrying them anyway!
How many tanks have you had in your car at one time?