The Bikini 
The Bikini is a super handy piece of formed 1/8" steel, it is exactly the same as the Bra butt without the battery riser. This allows for 6 volt battery application, different battery set ups and a storage area for magneto bikes.
Available in Raw mild steel or Stainless.
It supports many things:
Rear of oil bag
Mounts fender
Mounts seat springs, or not! (Studs welded 6" on center)
It does all of these things without being bolted or welded to your perfect OEM factory frame.... or sketchy chopper frame.
Every frame/fender/seat set up is a little different, so you may need to bend it or twist it a little, its pretty easy.
Comes with all hardware needed to mount including 4 spacers to fill space.
Get the optional battery belt and or Battery cover standoff to complete the kit.
Secure your battery and hide its existence with a nice stock cover.
If you run an extended front end 4+ over and run a solo seat with springs, get the seat spring studs Not welded on, then you can weld them at the correct angle after mock up.