The volts going into the amp is not what you measure. You check the voltage at your speaker terminals without the speaker hooked up. You get a -5 db test tone and play it at the max volume you plan to play your hu at. The voltage should be 46.9 for 550w at 4ohms.
Looks good man, I can't tell you how many aweful first boxes I've seen and that one is nice. If you haven't cut the hole don't bother with a terminal cup. Just get some ss bolts and use that.
Pretty cool trick but the fly was never dead. It's described as a coma or a deep sleep. Flys can do it in times of extreme survival. Kind of like a frozen bug comes back when warm.