I had the Quick Tank on my '06 GW. Bought used for $1k with all Marine grade fuel lines and Pingle valve in 2007. Expensive yes, but best investment I ever made for the GW. Fuel line taps into the stock fuel sending unit plate. Before removing the plate make sure the fuel level is under a quarter tank or risk a fuel bath.
I believe Boyd Welding does tanks for a fraction of the cost for Wings. Not sure if they've got a template to build off or you provide your own. Or heck, get a spun aluminum tank on Amazon for $200. Drop it on a flat metal panel and drill/attach ball mounts. Geez, $2500? Really?