I needed this and have seen others asking about same, so this is my design and hopefully someone will have a use for it. These were laser cut by Pololu.com from 3mm ABS. There’s a bit more side play between the slots/tabs than I expected but it’s functional. The tray is designed to be cut to custom lengths, so there are perforations every 20mm that outline the tabs to help with this. Use with 3mm threaded rod, nuts, and/or standoffs.
I tried to share the DXF design file here directly but the system won’t allow it, or any of the other formats used by Pololu for that matter. Would be nice if that were allowed.