This is an OpenSCAD script that can create custom trays based on the dimensions you set. If you're new to OpenSCAD this might be a nice project to examine. It uses the BOSL2 library, and it's a pretty simple and (hopefully) easy to understand script.

I tend to use this to create small “drawers” to fit on my desk, or under my monitor stands. The large one pictured is 185x185x40, but the idea is for you to make them whatever size you need as it's a parametric design.