I would definitely use a thin scraping of filler, Dave......that's what I did on the flying bridge of my clyde lifeboat, and gives you a reasonable surface..........I wouldn't even contemplate trying to get a decent fit with styrene sheet, myself.
once sanded down I used filler putty compound to get a very fine and smooth finish, using wet and dry paper, wet with a little fairy liquid on the warm water, using 320 and then 800 grade paper.