Edit: Now I've got an idea!
A cheap plastic cutting board is just the right size after chopping the "handle" off, to mount that 1 gallon tank. regular coarse screws will self-tap into that kind of plastic if you drill tight holes for the screws. (I've done it before.)
If one corner is drilled for the Dax pivot bolt, and the other corner drilled for 2 holes to mount to the stock holes opposite the engine (drilled on both sides for forward or reverse mount, how nice), the board can be mounted square with the drive channel, so it hangs down from it. This will put the tank in a pannier like position, and let it rest against the right tension strap for even more support. (spacers or washers required to bolt it to the engine pattern holes. This will also protect the backside of the tank from anything thrown by the wheels. Pictures to follow after parts arrive. Yay! This build has been a lot of fun.
I don't know how I could extend the dax channel without causing drive wheel skipping or strain on the frame. I am planning to just run this for now, till I figure out another option. I'm thinking of one of the 4 quart flat tanks like this: http://www.usplastic.com/catalog/ima.../sku/8663p.jpg
With 1 gallon flat tank, plus the stock tank, I can go 100 miles with up to 20lb cargo and I will be able to put my lunch cooler back on the front rack.
I will have to wait till I get it to see the size of it, but I may be able to fold a piece of sheet metal over the top tube, and clamp it together with the mounting bolts for this tank, so it sits center frame, near the seat tube. That would balance well.
If it can hang by the tab on one side without busting off, that would make mounting a breeze! I could mount it to the u-bracket or the side straps , esp the small ones like 2 quarts. I'll call them.
Here's a cheap option for a 1 quart booster tank
go ped tank $21 shipped with grommet and lines.
O-reilleys has these for $10-15
$15, 24 oz, can mount straight onto tension strap in vertical position