I always use 1/2 or 3/8 marine grade plywood that has glue not effected by fairly high moisture content. I after completion seal every surface with epoxy or at times spray with sealer/clear coat. This usually does the job. I never use particle board or MDF, moderate moisture content will warp and swell it. Phenolic is nice, expensive and don't breath the dust or it will mess you us pretty good.