And here I thought you were talking about the rare ACV Spare tire auxiliary fuel tanks sold for VW rabbits in the early 80s. 
I've seen an ACV spare fuel tank or BMW once while searching for my ACV spare fuel tanks. 
The small gas cans that fit inside the rims are available on Fleabay most of the time.. about $250ish. 
But you may run into a problem as the Vanagon shifter box comes down into the wheel center a little. Some clearance may be needed.

To the fella that transports a gal or 2 under the back seat NFW.. 
sorry that's worse than bungee tying one to the overhead roof rack. 

Enough Vanagon 'experts' get their safety panties in a knot when I tell them I've a 2nd fuel tank in the spare tire well
But the added 11gal saves a lot of worry some trip issues/interruptions.. sure I can find gas every 200miles 
But I don't always want to or even like to where the issue comes up. So flip the transfer switch and up-to 11 Aux gals. 
I can decide when/where I want to stop/fill up. 

Search fleabay for "Reserve spare tire gas"
http://www.ebay.com/sch/i.html?_trksid=p2050601.m570.l1313.TR0.TRC0.H0&_nkw=reserve+spare+tire+gas&_sacat=0&_from=R40

ACV Aux spare tire space fuel tank
http://forums.thecarlounge.com/showthread.php?6090420-FS-Super-Rare-ACV-Auxiliary-Fuel-Tank-for-the-Spare-Tire-well-350-OBO
http://forums.vwvortex.com/showthread.php?6090420-FS-Super-Rare-ACV-Auxiliary-Fuel-Tank-for-the-Spare-Tire-well-350-OBO 

google images 
https://www.google.com/search?q=ACV+spare+fuel+tank&hl=en&source=lnms&tbm=isch&sa=X&ei=iSsjU6ODIqfV0QHDq4AI&ved=0CAgQ_AUoAg&biw=1024&bih=1176

BMW version
http://www.sekaimon.com/i111208516195