Hi everyone. After a few researches and some help on http://Hackaday.io for understanding gel batteries, I decided to get my stuff together and buy the components for this project. So far, I've tested the amplifier with the speakers, the whole thing on battery with the smart charger and there doesn't seems to have any problem. I wasn't sure that the amplifier could take more than 12V but after a few researches, I found that it can in fact take between 8V and 18V : the amplifier chip (AN7190NK) can take between 8V and 18V and the rest of the amplifier is powered through a 7805 voltage regulator so there should be no problem !
I've been taking measurements of the components and made a quick 3D model to see how I can fit everything in the case and I may be able to come out with a solution.
Next step will be to drill and cut in order to install the amplifier (the front panel will be on the side of the case), the speakers (on top of the case) and also to put a connector for mains for the smart charger (I don't like to have a cable hanging around so I will cut it and just put a nice connector). Pictures will come soon too. Keep tuned !