I have the Hoerner wing tip and also not sure if the following is the "wing tip bubble" that you inquired about but here is another possible option to consider:
IMG_6896.jpgIMG_7003.jpg
As the picture shows I attached the "bubble" to the outside of the tip with Hysol and rivets. I had to trim the inside edges of the bubble before attaching it to the tip in order to get a better conformity to the wing tip at the interface. The raw untrimmed bubble didn't conform well to the tip contours, but after a lot of trimming I finally attached it and blended the interface of the two with filler.