There were hundreds of 20W 9cms PAs doing the rounds at rallies three or four years ago. They were made by Stealth Microwave and run off 12V and were selling for quite low prices. Dealers couldn't sell them, I think quite a few were bought for their attractive purple heatsinks. They came in a box with two and a PSU and diplexers that make good filters typically £25 or less.
It won't help to point out you should have got one while they were cheap but on the plus side, there are probably a lot of them still about.
While you look, there are also 50W models of that PA and some 15W HP amplifiers which were more efficient. The gain is about 50 dB so you will need an attenuator.