While beer can shims probably work, I like to buy cheap feeler gauges, and use whatever leaves you need to. You can try one, and after it's tightened down again, you know much more easily which leaf to use either thicker or thinner.
Do the best you can for planning and placing circuits for what you think you need......BUT........put in a sub panel with more spaces than you think you need, and put in a wall panel right below the box that you can remove-like plywood held in place with screws, and a space below that that you can also remove (or hinge out)as a place to put any future receptacles that you might need. You could possibly hinge the whole thing below it, but it needs to be so you can add boxes into it later if you need to. This greatly simplifies adding other circuits of any size later. It doesn't scare me a bit to run a 30 amp tool on a 50 amp circuit.
It's a great idea Trip...unfortunately mods don't have the ability to pin threads...I don't think so anyway. Marc would have to do it, and he's apparently still on turkey vaca. I'll bring it up to him once he's back in wood world.