I thought I would share what I did to mount my 1/2cf DI tank to a Magna folding cart.
I used a holder that is designed to hold dive tanks in boats. It works really well. It is super secure and easy to remove the tank. I can remove the tank and fold the cart flat in 10 seconds (not that I really need to).
If I remember correctly I paid about $50 for the channel and a pair of mounts from a local dive shop. I also used t-nuts, stainless screws and a piece of aluminum that I bought at Home Depot.