I've now seen fiberglass tips attached via 2 different methods. I think the more common method is to attach an aluminum flange to the aluminum part and then make the fiberglass part meet flush against the aluminum part (in your case, the rudder). If using this method, you may want to add a shim under the flange so that the fiberglass part can be a bit thicker than the rudder skin and still sit flush.
The other option is to just tuck the fiberglass part under the edge of the rudder skin and accept that the part won't fit exactly flush because the thickness of the aluminum skin will sit on top the fiberglass tip. I recently messed with my vertical stabilizer fiberglass tip and realized that the original builder used this technique. I have to admit that the thickness of the skin protruding on top the fiberglass piece wasn't (isn't) objectionable at all and is actually hardly noticeable. Using this technique, the fiberglass piece just needs to extend far enough into the vertical stabilizer to allow for fasteners.
No matter which technique you choose... an innie or an outie, I'd suggest installing nutplates for ease of access/maintenance in the future. My rudder bottom has a tail light in it and the top of the vertical stab has a strobe. I just recently replaced the strobe and I'm happy that I didn't have to mess up the paint on the VS to replace the strobe.
Here's an example where the fiberglass piece sits inside the Vertical Stabilizer. There are nutplates installed inside the fiberglass piece and the VS skin is dimpled to accept normal flathead (flush) screws.
Brighton, Colorado (CO12)