Use pegboard for the top of your cab if you're going to use it. It's flimsy stuff, so using the smaller area at the top of the cab won't compromise the stiffness of your cab at all.
Not that a pegboard back would in any way be flimsy, I'm just saying that it'd be a better place to use it, and use a solid back to strengthen the whole shebang, while the holes in the pegboard would allow the heat to escape in the most logical place in the cab - where it will naturally rise to.
Alternately, you could use a solid piece of wood for the top, and tape a speaker grill pattern to the top and drill the holes individually. Of course, you'll need patience and you'll end up with one wrist that's twice the size of the other if you don't alternate hands
