General

• Wire gauge: 18-15 for speaker cables and 15-13 for power cables. (Saturnus)
• Water proofing: I'd strongly advice you put your amplifier in a closed aluminum case with at least some effort made to make it water proof, such as Hammond 1590N1. if you're playing a late night/early morning during the summer the dew can visibly condense on the amp and drip off if not in a case. You'll want to avoid that.
• Choosing only from Parts Express parts the Goldwood GW-1058 as used in the North American version of the Boominator (because the p.audio HP10W is unavailable in North America) is an excellent low cost driver that pairs well with a superpiezo, for example the new GRS PZ1005 available from Parts Express. Use a 47ohm carbon composite resistor of 2-3W rating in series with the piezo. Must be carbon composite or metal film, even so-called "non-inductive" wire-wound power resistors have far too high inductance. (Saturnus)

Charging

• Running the battery charger while you are drawing power from the battery is not a good idea for a number of reasons. Wire a switch that when in one position charges the battery and runs the boombox off a mains-powered regulated supply, and when in the other position, the mains power is off and the amp draws power from the battery. (Buckapound)
• If you want a charger to be connected while playing it must be a charger that is specifically specified to do so. (Saturnus)

VU Meter LEDs

• Danish builders have found that with music control the LEDs will consume about 1.5-2W. It will naturally depend on the setting, the controller and the LEDs themselves but about 1/6th of maximum seems to be the consensus. (Saturnus)