One of my goals in adding electronic "harp strings" to my mandolin is that I don't want to change anything about my normal mandolin technique when I'm not using them. I've been playing for decades, and like how I normally play. This means I can't place these "teeth" anywhere my hands normally pass through. With a bit of testing, I think these are the available areas:

I don't normally pluck the strings that close to the fretboard, but after some experimenting I realized that I do sometimes when I want a different sound, so I need to keep that whole area open.

Since I think I want to be able to pluck a tooth on the downbeat and then strum on the upbeat, the area above the neck sounds like a better choice than below. I made a cardboard template so I could get a sense of whether this got in my way when I was playing:

It felt good, so I used my jigsaw to transfer the outline to the piece of 22ga (1/32") steel I'd attached my first two teeth to:

I cut a bunch more sections off the aluminum offset angle I used before, and laid them out in a potential layout. FYI this isn't the layout I ended up choosing:

These are all still pretty rough, so I filed off the burrs and rounded the corners. Then I epoxied on the piezos and went to bed:

Today I cut out pieces of sorbothane and glued the teeth to the tops:

I decided to go with an offset layout, and will probably map it as Wicki–Hayden (with the up stroke a half step below the down stroke). I glued the teeth to the backing:

I taped the backing to my mandolin, and played with my normal technique for a while to ensure it felt ok. No issues!

Next comes picking out a microcontroller, sorting out the circuits if I can't just go in direct, and a ton of wiring. [1] Then software to interpret the signals and generate MIDI, and integrating it into my existing system. Then learning how to play it!

[1] I haven't figured out my wire routing yet. Possibly I'll use a drill press to make a bunch of tiny holes, one next to each tooth, and do all the routing on the back? Though deburring the tiny holes on a sheet I've already glued the teeth to sounds annoying.