Insulating the hull questions.

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Kent, Thanks. That's a great offer!
But let me check out the Home Depos here first, maybe they have the stuff.
Maybe not a whole 4X8 sheet!
(It looks like Plas-tex could be glued on both sides of plywood before cutting pieces out for projects - cancelling a number of finishing steps.)

Still have 4 large spaces left to do on the Q-berth hull-sides... to insulate and cover. (Large pieces with a lot of compound curve, since the berth is under the starboard seat locker in the cockpit, where the hull gets pretty tricky.

Was sort of kidding about softening the panels, don't know if I have time left to start experimenting.
Brochure illustrations of Plas-tex show a sort of ply composite construct that might not allow softening. Steaming is just an idea to make sustained/controled heat in a wide box. Gas or electric would do just as well, if you happen to have a big convection oven at the boat! What temp to bend, not deform? Probably somewhere in the 350degree range, depends entirely on the actual plastic composition.
I'd start experimenting with a flat pan and boiling water, just to see if a tight fit piece of Plas-tex sheet would hold a mild compound curve. Armatex says their ensolite is good up to 220degrees "intermitant". Hot Plas-Tex might be formed in place, pressed on top of previously installed foam. BS only if it doesn't work!

Flat dimensional sheet won't conform to compound curves, even at less than 1/16" thickness.
The floppy vinyl sheet I've used to cover foam won't be pushed into any but the slightest hull concaves.
Instead, I was going to try some nice beige 5/16" cut-nap polyethylene (underscore) vehicle liner.* It's like a coarse polar fleece that has a filled water-resist under side that allows trowel gluing. Velcro hook&loop preferable for removal.. Many fleece fabrics can be cleaned by hosing, dries quickly.
This stuff, instead of having thin stiff vinyl or hard rubber backing like carpet material, has thickened integrated gluable backing of the same material that is soft & pliable - that hopefully conforms well to concavities - as I suppose it's meant to on auto doors, trunks & foot wells.
This liner on top of ensolite foam would make for a cozy berth.
Possible it could become home to damp mold and bugs, where impervious Plas-tex would be the correct easy to keep clean finish in that hard to ventilate corner of the boat. If it can be formed.
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*There are some SailRite youTubes to look up. One is: Installing Carpet Style/Hull Liner in Boat. There it shows a very casual installation of some hull 'blanket' on the overheard in a large boat V-berth. The blanket is an automotive type, a kind of slightly stretchy non-sided hairy felt. Relatively easy to put up using 3M spray adhesive and a remover for the installer's unavoidable over spray. Demo fabric is designed to be glued in.
Another video shows thin plywood furring strips on an overhead used to velcro thin panels (your choice) precovered in fabric (your choice) - with HomeDepo construction hard foam board filler between strips for added insulation. Both these are presented as DIY instructions.
Funky methods with informative & apologetic camera voice commentary. Ideas to adapt (or do better!), with Sailrite, no doubt, as the materials source.