Replacing cab mats with carpet - advice please

[Titan]

I am about to embark on recarpeting the front cab area of my van. I have ripped up the filthy orange carpet that was put down in 1986, and have taken up the rubber mat from the cab floor.
Now I am wondering - do I replace the rubber mat and carpet over the top (like they did when converting the van) or would it be better to put thermomat insulation down and carpet over that? My only issue with that is what to do at the edges where the footwell meets the cab step? The rubber mat has a nice lip that goes over the edge. How could I make this edge look neat while still being able to pull it up to access the screen wash filler cap.
(i.e. I can't glue it down).
Advice please! With pictures if possible! I don't want to mess this up.

Thanks.