I saw Lionel's external tank. Cant remember how much he said it cost but it looks very similar.
The tank in that vid is a Tanji/Tangi Tank and sits in the place of the pillion seat via a simple though tender mount bracket.
Lionels bike is an ex polite unit and still has the pod in place and was required to accomidate that pod to stay in use.
Hense the tank was built to wrap around the pod to some extent. Its not the capacity of the Tanji/Tangi due to that pod but it did fill the spec.
It does have a flat top for other luggage. The mounting is not at all the same though both units have plumbing in common and enter the main tank as normal.
I've fitted up plenty of tangi tanks and only built the one Lionel unit.
Cost is nuts if it achieves the desired result. believe it or not more fuel maximises rest time and alleviates that stress related to "what ifs the servos closed"...