Step 1: Measure and Draw
Figure out how tall you want the top of the tool box to be by standing next to your tape. Or you may want to determine how tall the base of the box should be, if the work area is more important to you than whatever you keep in the top of your tool box.
I used re-claimed birch-vaneered 1/4" plywood and (mostly reclaimed) 2x4 lumber sawed in half. So not only was the project cheap, it helped me clean out my garage. You can use 2x2 lumber or whatever you have handy. I suggest you take whatever looks right and use something about half as thick. It's amazing how reasonable 2x4 lumber seems when you're drawing something on paper. But be reasonable. A 2x2 frame is really strong---enough for any tool box I've seen.