The idea is to pack the mixed material very tightly around the areas to want to bind and lock together. I like to wet it a tiny bit with isopropyl alcohol. This thins it a little and lets you really mush it in there good. Let this harden completely and check that things are nice and secure. You'll be torqueing on this structure soon, so it should be solid.
Bit o'trivia: A number of props from the film A Nightmare before Xmas were sculpted and carved from propoxy. Very handy stuff.