Thanks for this overall view of a workable setup. I do already have the amp and the batteries in my shoulder strap bag - backpacking is a bit frowned on over here (France) in marching bands though some in Holland actually have a mock back pack - civil war style. The first time I tried out this idea I used my Ovation with everything else - including the speaker - in a back pack, way back in 2002...
I may try out a system with a speaker on each side though it will need to be more or less fixed using maybe a rigid harness like the bass drummers or some of the more modern multi-tom percussion players. I have been able to cut out all feedback by fixing an aluminium plate to the back of the guitar and fixing the speaker box to this plate at about one inch from the guitar body. The fixing is a large glued rubber pad 5mm (about 1/4 inch) thick. I'm still rather attached to the idea of my "autonomous electric guitar" as a single instrument even if I insist on the idea that it can easily be reduced to a "normal" electric guitar...
I've ordered the Jensen speaker so I'll soon be in a position to judge what my existing amps push out with a good speaker.
I'll keep you posted
Keith

Thanks for getting back to me - I was joking about the 76 trombones - I was quoting from the song in the musical and the film, the Music Man (1962) - I'm old enough to remember it though there was a TV remake in 2003.
I only have 8 trumpets, four bugles, two trombones and 8 saxes against me + all the percussion: 6 snares and two bass drums...
My first prototype was an acoustic guitar I cut part of the back off, put the speaker behind the sound hole and the amp and battery inside. Very neat but feedback at about half volume.
I now have the speaker in a separate box attached to the guitar (imitation Stratocaster) but using insulating rubber pads. The battery and the amp are in a smallish squarish bag with shoulder strap - just like what marching band players keep their music scores in.
I want to keep it as neat as that - just going up to 10" for the speaker - or as you suggest, putting in two. That could be neat too: one below the guitar and one above or at the end. I'm going to get me an odd shaped guitar like a flying V, a Dean V or something like that so as to fit in a couple of speakers more easily.

Well... I'm "competing" with a whole marching band... Obviously I'm not hoping to outplay 8 trumpets or 16 clarinets (not to say 76 trombones...) and if this works I'll be building at least four units which will even things out a bit. This is not the only use of the setup I'm building though the prototypes are already being used in this context.

I'm thinking of buying a Celestion Greenback 30 watt CEL-G10 GREENBACK 30-8 OHM to hook on to my Peavey 12v (20 watt) amp
I'm hoping there'll be advice from the forum about the models to try - bearing in mind that my budget is fairly tight and that weight is a factor.

I believe that Neodymium magnet speakers are lighter than most others. Are there any reasons for not using them?

"450 and 4500 mean about the same in the PMPO world, that's to say, nothing."

Oh dear - seems I'm in a different world. How can 450 W and 4500W = 15 W RMS if they mean nothing. I'm sorry I just can't follow that. Figures have to mean something to me.

I want to use this setup for an electric guitar but I'll also need a similar setup for a synthesizer and an electronic drum set.

Unfortunately, although I'm quite capable of building a kit (despite my 71 years...) I'm very hazy about matching Ohms, knowing what preamp to use (or even whether to use one...)
I really need a list of things to get:
What preamp - if necessary (12v)
What amp or amp kit (12v - 30/40/50 watts RMS)
What speaker (10")

I've read all that interesting discussion in the Watts -v- Volume posts and I plan to put in the best quality 10" speaker I can afford. Hopefully this will improve not only the sound but the perceived volume.

is that 4500W or 450W ?
I presume I have to use a preamp to drive a car radio amp? What would you suggest?

I'm new here so I hope I'm not asking a question that has been turned over many times already.
I need to build a portable solid state 12V amp 40+W feeding a 10" speaker that I've built into a separate portable round box. I've already stripped down an AmpCan (20W) and used the separate parts as a portable unit - I've also done the same with a Peavey 12V unit. I need to up the volume and limit feedback...
I wondered if using a readymade 12v car radio amp with a preamp would do the trick?
I'd be very grateful for any ideas
Keith