Those FRP panels are great! I've used them many places in my boat. They even hold up outside quite well. I used them on the outside of my propane locker I built about 12 years ago. They are holding up just fine.
If you're not familiar with them, walk into the bathroom of your local McDonald's Burger joint - that's what's on the wall. You can get them at Home Depot for about $28. for a 4' x 8' sheet. They're about 1/8" thick and bendy enough to take most compound curves you will find on the inside of a boat.
I have used them on the inside of my boat also. Here's a link to the "How to" I have on my Frugal Mariner web-site. Just look at that page though - in some of the rest of it you may be encouraged to buy stuff. (I'm not selling the stuff - just affiliate sales - so you moderators can decide if I've crossed the "no advertising" policy line. If I have, I'm sorry, delete my post and I won't do it again.)
The Frugal Mariner: Advice for the cash-starved boater -- Insulating your boat