I added an adjustable-height side shelf to my mobile workshop. It's held in place with sturdy 14" steel brackets from Lee Valley Tools that fit into 24" heavy-duty double-slotted standards, also from Lee Valley Tools. I adapted the shelf to fit my miter saw and added brackets to support workpieces. I can also set the shelf even with the workbench worktop to create a large assembly table.
The shelf is 5' long (the length of the worktop) by 16" wide. I used 3/4" plywood for the shelf. To stiffen up the shelf, I screwed and glued a 2x4 between the brackets on the underside of the shelf.
I also adapted the brackets so the shelf hangs from the workbench when not in use.