If you want something 'custom deluxe' in metal contact Fleetwood Metal Box in Colton, California (right off of the I-10). Talk to owner Larry Lord, tell him Lucifer sent you...
Where are you?
I an old RV furnace that would be PERFECT for this.
(I could MIG the three holes and drill mounts for a burning orifice of your choice. A grill burner works pretty well...and use an "INNER OUTER" steel pipe.)
Could be $$$$$$ to ship it once done though.
(I am presently in mid-atlantic region)
PM me if need be.
Open a door or window. Presto! The box (vehicle) is ventilated.
Actually, that's only partly a joke. Chances are, your vehicle isn't air tight. There are often gaps in the weatherstripping, usually at the bottom. Also, your climate control vents are usually open to the outside.
Nonetheless, you could probably repurpose a plastic wastebasket to serve as a vented box. Use something like a baking sheet as the bottom, place the tank on it, upend the wastebasket over it and duct tape it to the baking sheet. Make a hole in the baking sheet and run a piece of hose from that to... the gap in the door weatherstripping.