I made one thats 15 gal. fits in the tool box. after 450 miles I gota stop for a potty break and I still have lots of room in the tool box. used a fuel shut off solenoid,holly pump and an old filler neck cost under $200 to make it

 

if you have any chance of being inspected (commercial hauling or some states just like to pick on truck owners) make sure the tank is DOT approved. We run 8 Transferflow tanks at work 4 replacement tanks in GMC 2500s and four aux tanks in F450s and the F450s have to go through the scales, so they get visual checks at least annually.
BTW, never a single problem with the tanks or their electric tansfer set ups.