Whiteboard Tray, modular, parametric

I recently bought a whiteboard to help me keep track of how little actual work I'm getting done, and how much time I've recently been burning up making things to post here. It came with a tray that will barely hold five skinny pens and an eraser, and it has no sides making it easy to knock things off. So I've burned up a bunch more time making a modular tray system that expands to the whiteboard's width.

This tray will work with any whiteboard that has a frame made of a channel material whi