Just scored a 16"x14"x6" from eBay for $70 including shipping. I think that should work - it's Nema 4X, rated for outdoor. I am planning to tap a 120V leg off the 240V for pump and aux power.
Chosenwon, I could just as easily use that for the PID too, but I don't see a good reason to go one way over the other. I was only wondering if you specifically knew of a fault in the PID design that would prevent 240V operation. And FYI, cell phone chargers use a "wall wort" because they actually need DC power, not AC power. The wall wort is just a transformer/rectifier. Not sure of the internals of a PID, but it sounds like that actually can operate on 240V.
Regarding a 240V GFCI - correct me if I'm wrong, but a straight-up 240V GFCI breaker will adequately protect 240V circuits, no neutral line needed. However, if you want to use the 240V supply to power 120V circuits (using a neutral tap), that's when you need the GFCI with the neutral line, otherwise called a 120/240V GFCI. This is what I plan to use.
Overall, I'll have 2 PIDs, 2 SSRs/heat sinks, 2 240V (3-wire) outlets, 2 120V outlets, 2 120V switches, some terminal/fuse blocks, a 240V/50A GFCI breaker/panel... that's about it from the top of my head.
I'd like to fuse the individual components as much as possible, internal to the box. I'm looking for a decent, compact fuse block that I could use as a terminal block and fuse holder combined, preferably to hold standard glass tube fuses. For the big breaker, I may just stick a spa pack on the back of the box, unless I can think of a neat and subtle way to mount a stand-alone breaker internal to the box. I don't plan on having square-D rails in the box, or anything crazy like that.
Thanks for all the help everyone. I'm trying to document the process fairly well as I go along, so that when I finish I'll have another successful project to post up here for other's to use for ideas and to spur dialogue. It's been fun so far!