Frontrunner makes one.
https://www.frontrunneroutfitters.c...el-tank-left-side-land-rover-defender-90.html
I have zero experience with this but their other products have never failed me.
That looks like it is for a TD5 with a rear mounted tank. I'm not sure who makes something for a D90 with a side mounted tank. I have a Safari Equip aux tank, but it is a fill through tank to the rear mounted factory tank.
Why couldn't a NAS 90 rear tank be fitted in a ROW 90 ?
What occupies that empty space in the rear of a ROW 90 between the rear cross member & the axle ?