Step 1: The limited work area
I've placed two rulers in the picture to give a sense of size.
Step 2: Filling the space
In this picture I'm playing around with an Arduino Lilypad. If i need this space for something else I can easily pick up the tray without everything on it and place it elsewhere within having to dismantle stuff.
Step 3: Reusing the space
Note that when moving and storing a tray I often need to do some stacking, which I'l unstack when actually using the tray contents. But I do manage to avoid having to individually relocate dozens of pieces.
Step 4: Where projects sleep
Step 5: Materials
If you find that you need a more solid flat surface consider getting metal cookie baking trays. They're likely going to be more expensive than plastic trays, but still relatively cheap (about USD $6 each). If you are reluctant to do your work on a metal surface you can cover it with contact paper.
You need to be mindful of the tray depth, and the size of the tray handles. In my initial tray hunt I came across some nice solid plastic trays that would have worked great but the side walls were too high. High tray walls can get in the way of your hands, like working inside of a box. Wide tray handles are not a problem per se, but they increase the amount of free desk and shelf space you need for the tray, and you are better off finding longer trays with short handles to maximize your work area.