I do some steel work from time to time, and some years ago I got a set of tanks. In my neck of the woods I'm allowed to own my tanks and just swap them for full ones at the welding supply store. I live about 4 miles from a welding supply store, but the problem is I don't have a nice pickup truck to fasten the tanks to. They are supposed to be transported tightly clamped in the vertical position with the valve covers on. This is very difficult if not impossible in most cars. Since my little Explorer Sport has a trailer hitch, I designed a little platform to carry the tanks suspended from the trailer hitch. It works very well.

Disclaimer: I don't know if this is safe. I don't know if this is legal. Other than plain common sense and some years of experience I have done no analytical engineering on this design. For this reason, I have included no dimensions whatever except obviously it is useful to build it big enough to hold your tanks, and an 'M' sized oxygen tank is about 9" in diameter. My tanks are smaller, as I don't use that much gas and I find the smaller tanks more portable, especially as my 20s recede in increasingly distant time.

Here are a couple of design concepts. The first graphic shows the entire design concept. The second graphic shows the concept of a chain clamp. This chain clamp design is very quick to use and once you get it tight (you have to wiggle the tanks a little to get the slack out) it holds the tanks absolutely rigidly.

Here are some photos. The first picture shows the tank frame on my little "welding table". Note the short piece of chain, eye bolt, washer and wing nut, all cheap hardware store items. Once the tanks are in the frame, I just pick the frame up with a hand truck - it sits on there very solidly - to move it, but unlike a welding cart I can take the hand truck part away to use it elsewhere. My tanks live in the tank frame, though. Here is a shot of the tanks in the frame on the hand truck. Finally, here is a picture of the tanks on my car ready to drive to the welding supply store!