I finished shaping and glassing my seat for my XS400.
I didnt have to weld any tabs for this bike...I used the existing bridge that connects to the frame behind the battery.
I drilled out two 1/4" holes for the 1/4" bolts that I'll be using to mount the seat straight through the seat and the bridge. The first couple of holes I drilled missed the spot....LOL...
but the extra holes will be covered with glass anyway.
I pulled the seat off and widened the holes on the cross bridge so that it gives me room to move the seat around before final tigtening.
I put the seat back on the frame and put the bolts through the seat.
I used 3 layers of glass matt and fiberglass resin to permanently attach the bolts to the seat.
The last picture shows the bolts running straight through the bridge. This makes a solid mount!
When I make the fiberglass seat pan for the foam and upholstery...it will cover the bolts that I just mounted and they will never be seen. I mount my fiberglass seat pan the same way I just did for the seat to the frame...but the bolts just go though the seat and the seat pan.
I hope that makes things clearer.
This is how I mount all of my seats and none of them have ever failed and they are invisible except for under the seat.