I molded my own removeable tip tanks for my MB-339.
I layed up fiberglass over about 2" of the wing tips to create a "cup" that fit exactly, then I glued that cup inside the molded tip tanks and created a small fillet all the way around.
The tanks attached to the wings with two 4-40 screws. I glued in a guide made from outer nyrod sleeve to keep from losing the screws inside the tank.
I'll look and see if I can find any pictures.
BTW, I made the plugs for the tip tanks (and underwing tanks) from PVC pipe.
Edit:
Found a couple of pics.
