Talk to MC (jeepsr4ever) here, he's an Omix-Ada dealer.
Ask for part # 5357373-L
You can see it here: http://www.omix-ada.com/Catalog/p133new.htm
It's designed for the oversize plastic tanks that they sell. There are also a couple of other options on the page for 20 gal tanks.
Bob
tufcj