Yet another partybox!

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Okay guys, I know there are A LOT of thread about boomboxes. Heck, most of them are about the Boominator, which I've naturally drawn most of my inspiration from. However... I'm not going to build one of those (money money money).
This year will be my first at Roskilde Festival and for this my friend have put it upon me to design/build a boombox. They believe that because I'm an engineering student with an unhealthy interest in audio, I'll be just the man for they job :scratch1:
I've naturally done some ground work before consulting you guys, but honestly, I'm not a pro, or even particularly experienced, in this kind of designing, so I'd love to have a "You're looking great duuuuuude" from some intellectuals, like you!

The design so far has worked out to be:

Two 10" HP-10W mounted magnet to magnet
Two PHT-409 horns
One 4S 10Ah LiPo
A TPA3118 amplifier
A 10W solar panel, with wires so it can be placed remotely
The solar panel will be regulated through a simple linear LDO regulator.
Some accessories like ammeters, a voltmeter, USB power output bla bla bla. You don't really care do you?

I'm going mono because I see no reason for stereo in this application, with correct L/R summing of course. For cabinet I've found that 50L tuned to 72Hz should give a nice 3dB boost at around 100Hz (Thank God for WinISD). I'm also contemplating going for a 40L@80Hz for the smaller size. This would be almost 4dB at 120Hz, just under 3dB at 100Hz. Heck, I don't know. I don't have much experience on this subject. Also the cab will be made from 12mm plywood, just like the Boominator.

Okay, that was the basics. Now the stuff I might need a lecture on... My plan is that to eliminate relatively complicated LiPo charging circuits the battery will be floated at 16.2V (4.05V per cell) from the solar panel. My idea is that if the battery voltage drops to lets say 14V, the current capability of the solar panel, around 500mA, will be maxed out and the regulator voltage will drop to some degree. This will basically give a ghetto CC stage right? Then when the voltage climbs back up and reaches 16.2V, if ever, it'll be a CV stage. I'm hopeful about this. I'm not going to make any battery protection circuits this time 'round, but there'll be an ammeter on the solar output and a voltmeter on the battery to give some indication of whether it'll blow up sooner rather than later.

Another thing is the tweeter. I've done some trawling in the Boominator thread and around the webz and people, including Saturnus The Man, don't seem too happy about it. However all I found regarding why was that "it sounds bad, get the 407". Now, this is not meant to be HiFi. It meant to be loud and low power. The Boominator uses the PHT-407, but in my design the tweeter wont be as close to the ground, and therefore I'd like a bit more dispersion than the 407 offers (60° vs 409's 100°x40°). The sensitivity of the 409 is higher than needs be, but that's always dampenable (this should be a word). Also because of the somewhat extended low end response of the 409 it'll need a different x-over. All this aside, why wouldn't I use the 409?

On an end note I'm having great difficulty finding a thinfilm 10W solar panel in EU. 10W Poly panels are easy and cheap at around 17 GBP each, but I know that in partial shade it'll be useless. Do you guys have a source or something for thin film panels? Size is less important.

So, Masters, what do you think? Am I just shitting in the nettles or might this actually turn out good?
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