We stripped the interior of our Captain
trailer down to the fiberglass. Then put in 3/8" ensolite closed cell foam (from a foam dealer in Kitchener - you can probably get it in a marine store too). On top of that we put boat hull liner which lets water run right through. Looks like corduroy (sort of). You can put velcro on it. We got the boat hull liner at a local fabric store that carries everything from soup to nuts. We didn't even know it was boat hull liner at the time - it just looked like the right thing to use. You could also use car headliner (more color choice) but I'm not sure how thick it is and it might be more "wrinkly".
is more "square" without the curved upper corners. However, you can cut a pattern. If you managed to save the old liner, you could cut it from that. We used paper patterns. Measure twice, cut once.
We also used the glue recommended by the foam dealer so we didn't melt the foam into a blob with the wrong solvent. They also recommended using a sprayer for the glue and be prepared to throw it out at the end as they are a pain to clean (so buy a cheap spray).
The foam application doesn't have to be perfect as you are covering it with the liner. I think the whole job once the old liner was stripped out took about a day. with the curves, it will probably take you a bit longer.