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?
The "Quick Tank" design allows for easy opening of the trunk, plus a flat area to mount tent/bags, etc... The QT is around $1500, and as I see it, the extra $1k is for mount adapters for the new GW design. Somewhere on the Internet, there was a chap building an accessory fuel tank for the '18 Wing. Unfortunately I don't remember the link.
Typical of Max to provide Overpriced gear. there is maybe $50.00 worth of consumables and the rest is a markup for the products Intellectual Property.
personally, i would just make some minor adjustments to allow those tanks to be mounted to the handrails. easy on/off procedure.