Wonder if you could make this tank work for an SX/4 The fill neck would need to be reworked and the sending unit does not have the return tube. https://www.youtube.com/watch?v=ajIBbQthyMw
Looks like it could work. Here are a couple pictures of my Kammback tank. You might be able to use the Kammback/SX/4 sending unit, or add a fitting for the return line. You maybe able the adapt some fuel filler hose to work with the inlet.
I've also included a screenshot of those new tanks. It would be great if this guy could provide some dimensions.
Thanks for the link!IMG_0223.jpeg