Having finished off a lot of my 6mm ancients I needed to maximise the number I can get in one box, Why carry two boxes when one will do, well it's lighter came to mind when I'd done, but I'd rather pump-lead that walk the distance twice!.
Anyway one 9 ltr really useful box can take 4 20mm trays and 2 30mm trays 6 trays in total for a total height of 140mm. As the MDF is 2mm wide then a 20mm tray gives you 18mm of model space, a 30m tray gives you 28mm
When I fill the RU box I put the Kontos carrying tray in the top s it has at least another 2mm of space because the lid has a raised centre.
In the picture the top two trays are the 30mm ones with cavalry in.
The white tape is masking tape I've passed through the handles and then stuck to itself, these surprisingly robust handle allow me to gently lower the trays into the box.
The 9 Litre box, yes I can still lift it up.