Sounds like you're going in the right direction.
The Galibier metal inserts are simply 1" aluminum angle strips purchased at any hardware store, cut to size, and screwed into the underside of the top shelf. Filling the sandbox is a several step process because you need to fill it about a quarter or third at a time, place the shelf on the sand, and whack the shelf with a rubber mallet to compact the sand. Repeat the process until the sandbox is full and the platform is level. This is what I was instructed to do by Thom Mackris.
The Galibier metal inserts are simply 1" aluminum angle strips purchased at any hardware store, cut to size, and screwed into the underside of the top shelf. Filling the sandbox is a several step process because you need to fill it about a quarter or third at a time, place the shelf on the sand, and whack the shelf with a rubber mallet to compact the sand. Repeat the process until the sandbox is full and the platform is level. This is what I was instructed to do by Thom Mackris.

