Blink - Just get regular plywood. There used to be a big difference in grades of plywood due to the glue used. That was years ago before advanced polymers. Now, plywood glue is all excellent. The difference in grades and costs comes from 1) The number of plys (layers of wood) - more plys = stronge plywood and 2) the surface quality. If one or both sides are nice sanded wood, as opposed to wood with tears and knots in it, the plywood is more expensive, eh. But you don't care what your box looks like so just buy the cheap plywood. Save your money for better lights, not better plywood.

A GFI breaker has two sides to it - line and load, eh. Line is the electricity coming into the receptacle - pretty straightforward. The Load side is where it gets trickier. Any receptacles that are down circuit from the GFI rec'le will be protected by the GFI breaker if they are connected to the load side. If you DO NOT want down circuit rec'les protected by the GFI (and there are many good reasons why you might not), then hook everything together in the box - black with black, white with white, run pigtails from your connection and hook the pigtails to the appropriate connections on the receptacle.

Simple enough, eh?

My Canadian is a little rusty but I've translated this as best I can, eh.