Rub rail replacement -- insert insertion?

Since finally closing the book on that pesky rolling idle issue I decided it was time to get started on the installation of the new rub rail purchased all :those months ago.
Gotta say it sure puts a smile on the face to be able to transform from this:







To this:








But my question for the group is how the heck does a guy get the insert inserted??
I've googled, read and watched videos and tried much of what was suggested but thus far without success.   Is there a special trick to this I have yet to discover or is this just as much of a pain for everyone ....   Suggestions?

      

Well, there's a thought - heat both. Now all I need is a nice hot day ... without the thunderstorms and rain.   Funny, the old insert came out FAR easier than it seems to want to go in   

Well no sun here ... In fact late this afternoon we had quite a storm, complete with 2 inch hail and a ton of yet more rain.   On the bright side I'll get those few little dings on my truck taken care of now    

Anyway, after getting home and surveying the damage I decided to immerse myself in the rub rail and indeed heating both was what was needed, getting half the starboard side done before stopping for dinner.   I'll easily get the rest done afterward so thanks for the input guys!