wow, 2" gap. Lots of options for you! I'd probably use that pink or blue construction foam available as 4 x 8 sheets
etc at the lumber
yard. The other option is that "nylon" like packing foam... usually dark blue or white. Probably not as good a insulator but easy to bend.
Often Battens (plywood strips partially sawed on one side to allow bending) can be used to attach hard panels. These are usually glued/bondo'd etc to the overhead athwartships with some glassed on additional tabs every foot or so. Then you can screw the panels on the strips.
Spacing the strips for good use of panel sizes is important and you will then need to make teak
(or?) trim strips to cover the seams. Not a small project
There are "formica" like covered ply panels available that might work well. Not really marine
stuff though. Although I think they are meant for damp environments.
I did one boat with 3" wide varnished white cedar strips running fore and aft. The advantage of narrow strips over panels is to get at deck hardware
etc, you just have to remove a strip or two..