Use an under-the-saddle piezo. It goes in the bridge under the saddle. You will still have to have channels or access cavities to reach the input jack. If you use an input jack with a preamp built in (recommended) you will have to have cavity room for a battery. You will likely have to have necks and other parts custom made, in which case you might as well have the whole thing built for you. If you are interested, I have one of my own make for $350 with a hard shell case. It has no preamp, but there is plenty of room in the control cavity for any additions you might like to make.