Highest capacity Aux tank for short bed

[DHayden]

I have searched the site and havent come up with the answer I need. I want the highest capacity aux fuel tank for my DRW Mega Cab (short bed) that does not cover the factory pucks. What is the biggest tank you have found that will fit under the tonneau cover? Anyone that has actually installed and verified fit would be great. Dimensions, brand and model would be great also.
I do not want a tool/tank combo, fuel only for max capacity.