I struggled trying to fit the tail tip under the skin and eventually gave up.
I decided to do a common mod which is instead of trying to squeeze the tail tip between the skin and tip rib, just insert an aluminium “lip” and attach the rib to that, you then just need to trim the rib a bit more obviously because it does not go under the skin anymore.
Here’s are some pictures showing how this was done:
Many people have done this mod, also, I decided to rivet my tail tip to the lip but this mod also allows you to use nutplates with metal screws instead to attach the ribs to the tail, this way you would be able to remove the tips very easily.