I built some raised beds with a pressure treated frame and corrugated tin sides. I lined them with the heavy duty ground cloth available at Lowe's. In my case, the ground cloth was kind of creating a "bag" inside the pressure treated frame. If you want to keep the wood almost totally dry you might get some pond liner material and punch drain holes in the bottom. Cedar will last fairly long underdamp conditions, but it has a more rough texture than mahogany.

It's a fact that sometimes, raised beds are undone. I can't see that a layer of gravel will be all that much of a help for drainage (though I don't fully accept that it will harm it either.) But it would surely make it a lot harder to clean this project up when and if ever the time comes. I would go sans gravel. Also, I know you already have a plan for making the container out of wood, but it's the case that many wood boxes end up looking pretty shabby down the road. I recently came across a video where someone made raised beds out of interlocking precast concrete panels that they fabricated themselves. ((Link to precast raised bed panels.)They were much nicer looking than the typical wood affair and are also flexible in that they can be linked to create larger size beds. They'd be much more durable, too. If ever undone, they could be reused somewhere else instead of being pitched in the trash. No need to worry about rot or coating the insides.

Floral, thanks for your suggestion. I did look at Butyl rubber but that is just not considered dangerous. There's a bit to go to a safe solution. I need to touch and smell it to make a decision. I'm one of those people who cannot stand a 'new car smell'. I'm contemplating a simple wooden box standing above ground. The bottom will be furnished with something that provides a bit of slope. Drainage will be handled by one hole punched through the liner and drilled through the wood. I might choose to punch three holes and run a gutter underneath. So wood, something to provide sloping (styrofoam?), liner, coco liner, soil. That should be fine. If I can't effect this solution, then a large metal container with a metal pipe fed to the bottom and some kind of suction/pumping mechanism to reuse water. Ryan, let's not get too deep on this. This is just a small planter for my balcony. I have lots of room but I would like the ability to bring it indoors to start early. I live in Toronto and May can still be quite cold at night. The balcony door is just over 2 feet wide. So my width is limited to 2 feet and length will reach 6 feet max. I need to line it to: 1) not worry about the wood, 2) not stain the balcony and 3) reuse water full of nutrients. What about galvanized metal sheets? Let's assume these metal sheets are made in North America and not imported from Asia. Are they safe? Will they last? Will they have adverse effect on soil, such as overheating?

If you were the national of any other country, you would just do it the best you can, expect the best, and not worry about things not working out. Because you're American, you're going to worry yourself sick about every bit of minutia. I would bet you could attack this in 50 different ways and it would be just fine. Honestly, your description is complicated. It would be much easier to understand if you'd show a few pictures of each thing you talk about. I suggest you post these pics and you'll be more likely to get some good advice.