I have designed an awesome one stringed double washtub bass that really blasts out a deep warm tone. My problem is when there are many playing in a group it's hard to hear my notes unless I kneel down close to my tub openings. Is there a device that would incorporate a pickup-to-an-earphone without the bulky amp and power source. The sound quality doesn't have to be great... I just need to hear what I'm playing. I do not need or want amplified sound. My instrument is just fine "unplugged"
Cool design.. I have to say, I'm not an expert on single stringed, double washtub basses... but, What if you attaches a bucket or container up at the headstock with the opening pointing towards your head. I'm sure the neck is vibrating most of the note your playing. The bucket would vibrate too and probably make sound you could hear.
The sound comes from the resonators at both bucket bottoms and the enclosure acts as an echo chamber to marinate the sound. I guess I could configure the string into a Y vs. a T but I'm afraid it would be like cutting a 3x larger hole in a guitar and turning the hole straight up. I'd lose quality of sound for an increase in directional volume. I might experiment with the idea. I'd rather just find a way to discreetly electronically monitor the chamber sound to my ear without a lot of electronic hardware which takes away from the charm of the instrument.
What material is your string made of? Either way, I think your ideal solution centers around one of these: https://www.amazon.com/AP2AC-amPlug-AC30-Guitar-Headphone/dp/B00NAUHX1G I have one; it's very compact, and it runs on 2 AAA batteries. I use it for quiet practice sessions. If you use that, and some kind of ear-sound-thing (earbuds, open-back headphones, in-ear monitor, whatever), all that's left is how you're getting the sound into the headphone amp. If your string is made of steel, a cheap single-coil blade pickup like this one might be the way to go. Otherwise, a small microphone. Your bass looks really cool; I love improvised instruments. My college jazz bandmates probably still remember me as the guy who made a playable "instrument" out of a pizza box, two pens, and three rubber bands.
I seriously thought of this! Scottkarch's idea was better, using the fret board as the string though :{)