Festival Stereo From MAKE
Well, they don't actually tell you how to make it, some reader wrote in from Sweden showing off his latest creation. Although there are apparently specs so you can build you own in his Flickr account.
It looks like either a guitar amp or some other professional grade speaker that the guy built in a car radio and cup holders, for beer. I assume there is also a fairly hefty battery built in to power the thing, otherwise its portability is decreased to wherever there is an outlet.
But, supposing it has a battery inside, this would be pretty cool, I mean, it would definitely overpower your run of the mill boom box and it portable enough a couple people should be able to carry it. Plus you'd have use of the top as a table and the cupholders, that has to be the best part.
Festival stereo [via MAKE]