And I think I will wind up in that type of situation, but getting anything behind the back of the poly hull is really going to be a problem as there is no access so it may have to be an expansion type device.
I don't have a clue at this point but I think it's a single wall in the area I need to mount a custom plate.The existing threads into the inner hull for the rudder on the stern are only about 5/8" long in thread length.
Post some pics of your yak and the areas you want to work on and the "things" you want to mount for the best help. It help for us to see what you have to work with and what you want to do.