I used the Springtail Solution side rack.
That is awesome. Definitely a mod on the short list.
My frustration and excitement to use my tank this weekend got the best of me today. I went to the local fire supply shop and bought the box type vehicle bracket for $50. It's not what I want long-term, but it will get me by for the next couple months as I absorb the holidays.
I really like the C02 tank. Airs up my 37" tire from 10 to 35 PSI in 1:20.
Back to the mount:
I bought 3/8" bolts, washers, 1" spacers, nuts and lock nuts from Lowes this evening ($13.) Three bolts attach the mount on the side. For now two bolts on the bottom are there to support the foot. It looked way too difficult to mount bottom bolts through tub, too much in the way. In the future, I'll put a rubber foot on the bottom of those bolts so the metal bolt is not rubbing on my bedlined tub.
On the side, each bolt gets a washer, comes in the vehicle from the wheel well. The lining is very difficult to get under for the top bolt, but the bottom two weren't bad. When inside the tub, I put some RTV around the bolt then followed it with a washer and nut to secure the bolt to the tub wall. Then a 1" spacer goes on, a washer, through the mount, followed by a washer and a locking nut. Its very secure and I shouldn't have any problems getting by for a while. Tomorrow I'm going to cut the bolts short and paint them black. Then it's wheeling time Sunday!