This is a tray to place your Gameboy Pocket into when you are servicing it or disassembling to modify it.
The tray is designed so that the pocket sits on a slightly raised platform that means the case sits level and the buttons are not under pressure. This makes sure that the buttons can be installed with no concerns they were displaced - an issue that made me design the thing in the first place.
To help provide cushioning and to prevent any possible scratching of the screen I would suggest the addition of thin adhesive felt.
The author marked this model as their own original creation.