Concrete Forms for Small Project

I'm planning on doing a small project with concrete, and I don't have much experience with it so I could use some guidance.

Basically I want to make an open-top box out of a lightweight concrete mixture, such as with vermiculite or perlite instead of gravel, or I might even try some cellular concrete mixtures (aircrete). It will basically look like a large rectangular flower planter made of concrete, a box within a box for forming purposes, and it'll nominally be a monolithic pour.

With the lighter concrete mix, I think I'll be able to get away with some form material weaker and thinner than 1/2" plywood if I have a couple braces. Its total volume of lightweight concrete will be about 3.64 cubic feet, with 4" walls, reinforced with some steel mesh hardware cloth or something.

So that's where I'm kinda stuck. I've heard that plywood forms only last so long even if you sand them smooth and add a good release agent. I don't need forming materials with as much structural integrity as plywood, so I might be able to use something that is thinner but will last for many more pours. So my questions are:

1. How long do plywood forms usually last if you're careful about not letting your concrete cure for too long and use a good release agent?

2. Are there any thinner materials you can think of that would work as lightweight concrete forms, that might even be a bit more expensive, that might last for say hundreds of uses?