It’s a bit awkward, but with the roof up so the bin/binnacle/box it suits in behind the seats is empty, kneel on the seats and feel around the plastic trim in that area. You’ll find several overlapping pieces.
As the car flexes while driving they move ever so slightly against each other and that can cause a squeak. If you can get something along the edges - pipe lagging is easiest but is a bit unsightly when the roof is up - you stand the edges off of each other. Voila, no more trim panel squeak.
Getting draught excluder foam in is bloody awkward but looks better.
Worked for me in my old 06 NC1. That said, the noise was possibly indicative of a lack of stiffness in the chassis and I found that was due to rust pretty well everywhere!