Oh bugger I should have posted here. Oh well, copy-paste powerrrr!
As one of the testers for the underseat tank Rade (fellow inmate) is developing I have the honours of enlightening you with my experience so far. I received the tank last week and found some spare time to mount it today. It's probably fair to say the final version will look much better, this one is just built for toughness and testing purposes :)
The tank came with all the mounting hardware included, and it's absolutely bomb-proof.
It's going to take up pretty much the entire rear bit of the bike, so you do need to do some cutting in the rear fender. It's not a big deal, but you can't get around it. This isn't a how-to so I won't annoy you with mounting instructions, but it was pretty straightforward. It comes with its own petcock and all the tubing, just run it around the bike to places you want it to go.
I've only done 50 miles so far, but it made it safely across a dastardly tricky stretch of deceivingly flat highway! Maybe that isn't a very high bar to set, so now to give the thing a decent test for a few days; 2000 mile all-roadtrip starting tomorrow