Nice and clean Steve. There is no greater feeling than building something yourself. I'm sure after this your son is going to want to build everything. And as far as the hat box not being stout that's a load of crap the P3s will be totally fine and you also used 1 inch mdf.
Hey i have a Chevy Trailblazer and you have to buy the GMOS adapters depending on what model TB you have. You can buy the cheap Scoshe model if you want but i wouldnt recommend it not a very good quality product if you ask me.