If you use an epoxy resin it will be much safer than polyester resin, but much more expensive.

As for leaching, I don't know for sure because I never did the research.

I would be very cautious because the resins are toxic, so please do your homework before you attempt this.

Ron

Fiberglass would work well for what you have in mind. If you are not experienced working with resins and fiberglass materials, find a good local source who specializes in the materials and can answer your questions. A final coat sometimes called a gelcoat will make it waterproof and can easily have color added. I have an old wheel barrow that rusted through more than ten years ago which I lined with green fiberglass and use everyday. It is especially good for mixing mortar or cement as it is slick and washes up so good. Al

Thanks for the replys.
I think ive decided to go the EPDM rubber route. I have some experience with fiberglass, but in the long run i think the EPDM will be easier to use. I'll be constructing the box itself sometime this week. Tentative dimensions are 6 feet long x 32 inches wide x 24 inches tall.