Customizable rounded square trays, supports multiple cells.
Some example output stl files are included.
For round trays, see https://www.myminifactory.com/object/customizable-round-trays-18099
If you want to specify the outer bounds instead of the cell size (you want to use this as an insert), check out codeowl's version - http://www.thingiverse.com/thing:54599
Use the "Open in Customizer" button to make your own version. The changes will propagate faster at low resolution, but the resolution can be increased for final output.