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 which will accept a T shaped rail. It will not work with frames that do not have this channel (e.g., solid wood frame). This is the whiteboard I bought and used for the default sizing: http://www.amazon.com/Master-Boards%C2%AE-Drywipe-Magnetic-Whiteboard/dp/B00TQV2X8G

**Note:** The defaults in Customizer and the STLs I've included are optimized for a Lulzbot Mini which has a 0.5mm nozzle and uses 3mm filament. If your nozzle is not as wide, or you wish to use cut sections of 1.75mm filament