The tank has the standard cups that fit around the stock rubber bushings. There's a flat tab off the rear of the tank that i just hold down with the fiberglass seat. I have adhesive rubber padding that helps make it all snug. Sorry i dont have any pictures and the bike is sold now so i cant get any.
Basically i made an aluminum frame that is epoxied to the underside of the seat. I drilled 4 holes through the seat and said frame, then welded tabs to the frame that had nuts welded to those. 4 flat allen head screws then sandwich the rear gas tank tab between the seat and the frame. Then the seat pad velcros over the allen head screws.
Here's a pic of the aluminum frame before it was welded together, you can see the tab on the back of the tank:
I think most people use wood for the seat frame which would have been much easier, for some reason it never occurred to me to use wood.