Unfortunately a 690 tank is the
one thing I don't have so I can't confirm the dimensions either....
What I do have is an F800GS underseat tank, also plastic, and it's light, around 3.8 kgs I think?
The tank shape looks a bit weird but the subframe would be very simple to copy if you look at it. You can also get those really cheap if the side rail is bent like the one on the left. This happens all the time due to the stupid placement of the passenger pegs, but would be fairly easy to straighten and fix yourself.
