How about this one - a concrete block guitar
....you too can also have a concrete block for a guitar if you buy the special piezo bridge for £233.87 (http://www.astrings.co.uk/Lashbrook_Naturacoustic_piezo_bridge)
Or instead you could just build a cigar box guitar, stick in a piezo disc and have loads of cash left over ;O)
It would be interesting to see how it compares to a piezo disc.
Or, you could have Bairfoot Cajun make you a brick guitar: