Here is a rough (aka bad) video to demonstrate how it sounds with no effects:
Also, here are some references and inspiration:
Step 1: Materials
metal casing from hanging work light
1" piezo buzzer
thick felt with adhesive back
two different gauge weed eater strings (I used .65 and .80 that I tuned to E and B)
various nails and screws
a pencil split down the middle
As far as the paddle goes, I used a SeaSense from Wal-Mart, but I'd suggest to try to get one from a sporting or camping store mainly because mine seems flimsier than others of the same brand but from another store.
Also, the machine heads can be either for a bass or guitar; a guitar machine head can hold a piece of .80 weed eater string with no problem.
Step 2: Drilling and Stringing
My scale is 32" but in the beginning I wanted 36". That didn't work for me because there were dead spots that won't make sounds for some reason. So I have holes drilled 5 inches behind were my piezo is, but it still works fine.
Now it's time to put in the machine heads. Find an appropriate size drill bit and start in the flat part of the handle. This isn't an exact science, just make sure the holes allow you to turn the key of the tuner and that both machine heads are pretty symmetrical looking. Once that's done, just screw them in.
Tie your strings into tight knots and send the untied end through the holes, over the split pencil/future nut and keep them loosely in place by only turning the machines once or twice.
Step 3: Finishing the Headstock
Step 4: Making the Piezo Pickup
Next, get that thick felt stuff (like the kind you put on the bottom of furniture) and cut two pieces to around 2" wide and 1" tall. Sandwich the piezo in the middle of the felt (metal side up) and place the front of the pickup at the end of the scale length and put two small nails on both sides to hold in place.
To check if it's working, just twist the wires around the connections on the jack (polarity doesn't matter) and if it's working and making sound, disconnect the piezo so you can put the cover on that holds the jack.
i have to give credit to the guy who made this:
Without this felt idea, this would be a screeching/popping mess. Trust me, I tried.
Step 5: Body Work
For a strap button on the body, I used a roofing nail (be careful- don't split the wood if you do this) I had a hundred of laying around. For the one on the neck, I put a big headed screw where it felt right for the bass to hang and not be too in the way.
Step 6: Finished and Other Ideas
But if you'd like to draw in some frets or even put some in (ambitious!) here's a good fret calculator.
If you can't stop at just frets, make a fret board. My bass still has a spot under the E string that won't make a decent sound (I think there may be a knot under it or something). A strip of 3/4" wide wood or metal glued on or flat screwed down seems good in theory, but I haven't tried anything yet.
Heck, why do you even need two strings? Just put one on!
If you're a fan of upright basses, I've been thinking that someone could rig a fix topped guitar stand to hold one of these- that would be interesting.
Also, you could drill the string holes at the top of the handle and the machine heads at the bottom for more of a headless look and better balance (although the regular way has a good balance anyway).
Well, have fun and good luck with your new bass!