The trays are 14 3/4" x 8 1/4" and made from injection molded plastic. Gems On Display sells them in depths ranging from 1" to 4" along with carrying cases made from a tough lined nylon which will hold up to 12 - one inch trays.
I changed up the basic idea by lining the trays with magnetic sheet because I base my miniatures on steel washers. The magnetic sheet isn't remarkably strong but will keep the minis in place during regular movement and transport in the car.
Recently I stumbled on Dave's Baggage Train who appears to have commercialized this technique and offers foam inserts and other options.
As miniature trays these options work out perfect for all but the largest models.
Now that I am printing out Dragonlock™ terrain from Fat Dragon Games I needed some method to store the numerous tiles, walls, corners and other accessories. I decided to make a set of custom trays with just over 2" internal depth that allows for packing the most tiles in the smallest space.
Here are a couple finished trays filled with the Dragonlock cavern tiles. (I haven't finished painting these yet.)
The basic process is cut four sides from 1/4" plywood and cut out a rabbet along one side to hold the bottom then miter the corners. The long sides are 14 3/4" by 2 1/4" and the short sides are 8 1/4" by 2 1/4" so that the finished tray can still fit in the gem cases and I can pack up a case with a mixture of terrain and miniature for a game.

Aligning the points of the miter then using masking tape across the joint gives a strong clamping force and will keep the joint square as long as the miter was accurate.
Here is the inside of the joint ready for glue.
When the joint is closed up the tape holds the surfaces together without clamps.
The bottom then drops into the rabbet and is clamped.
I add small pieces of wood to the bottom (tongue depressor) so that the trays don't slide when they are stacked.
Once the table saw and miter saw are set up it is easy to make a stack of trays to store everything.











