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[BellyDoc]

I'm glad to see that you've mounted this vertically.

Once in position, a mounted CO2 tank tempts us to use it by just hooking up a hose, and opening the tank valve.

If you mount it flat, this is a DISASTER waiting to happen.

If you open a tank valve on a CO2 bottle laying on it's side, it can blow superheated liquid CO2 into the regulator, where it will decompress into gas, explosively.
On the driver's side, the rear corner window is invisible to the driver from any angle, so having the bottle upright does little to alter your view.

Nice job.

An option, if you don't find a sheet of thick rubber that you like for elevating the base of the tank off those bolts, is to use rubber furniture feet. Many of these are supplied with an adhesive backing already in place. They're typically a semi soft rubber or synthetic polymer. You'd just need to find a type that's slightly taller than your bolt heads.

You may be able to improve the situation by lowering the bolt heads too. Unless the holes through which the bolts pass are very loose, you can dispense with the fender washers on the metal side. I'd say that having a washer on the plastic side might help distribute the clamping force, but this is a comparatively minor component of your bracket fixation. The crossbar part of your build is more than equal to the task of carrying the load by itself.