The ATV people have adopted a kind of flat jerry can that spans the front or rear of those vehicles. They have a number of holes and attachments and are meant for the rough life that a tank like that leads (cap, filler, expansion, etc). They fit quite well in the bottom of the luggage rack and, strapped down appropriately, will not move. They come in a variety of sizes, so you can pick the size that suits your needs. As they are quite thin, you can still use the luggage rack for (some) other things. Search for Rotopax or SXMA. 

> On Jun 7, 2020, at 8:00 AM, Alex MeVay <alex@MEVAY.ORG> wrote:
> 
> If you have a 2wd, for a pretty slick solution, but more of a project,
> the ingenious PO of my van had added a second tank above the
> transmission.  I'm pretty sure it's this one, 6.5 gallons:
> 
> https://www.speedwaymotors.com/Black-Poly-Fuel-Tank-6-1-2-Gallon-8-x-30-Inch,5404.html
> 
> A second fuel pump fills it from the main tank (~5 min), and a valve
> lets it drain back by gravity (~1hr).  I upgraded the valve to a
> solenoid, so now it's all controlled by a single toggle switch from
> the dash.  No vapors, no leaks, hidden, >400 mile range if you're just
> cruisin' on back roads.  Further details upon request.
> 
> I was sort of wondering...I believe the syncro fuel tank occupies this
> same position above the transmission.  Has anyone fitted a syncro tank
> to a 2wd for massive fuel capacity?
> 
> Alex