This is a "Gugulele" implementation based on the fantastic Onshape FeatureScripts from Big1 (https://www.thingiverse.com/big1/designs).

Here my copy
https://cad.onshape.com/documents/5cb724bfe195206ea2a6bb41/w/8f57d00be456652e00fc144c/e/4f8aa722a2bfa2f93d6b63be?renderMode=0&uiState=664a504d53f2250ee94364d1

Body is printed in PLA with infill ~15-25%, while tuning gears and bridge are printed in PETG with 99% infill. Note this build is extremely cheap while still having good tuning capabilities of geared tuners.

Cost split should now be:

• ~300g of PLA (at 21EUR/kg, ~7EUR)
• aluminum spines 10x2mm: 2x~30cm (3EUR/m, so ~2EUR for this build),
• screws: ~1EUR (should work to lower that).
• ~20g PETG (negligible cost)
• (optional) ~10cm of iron wire: I have used hanger wire that I got for free (negligible cost)
• (optional) 4 x 1.5mm x 10mm nails (to keep the gears in place). negligible cost
  Total cost of this should be in 10EUR ballpark.

This is a sopranino instrument (scale 260mm ): it could be tuned like a standard ukulele (GCEA), but I feel strings are a bit sloppy, and I find better tune DGBE (like a guitar one octave higher).

This is my first "gugulele" make, so I have experimented many things:

• Scale 260mm: i went for the smallest scale that I have heard of. Even if I am used to the travelele (https://www.thingiverse.com/thing:2786864) sopranino scale (330mm), I find this one too small for my hands. Fortunately I had foreseen to give it to a young toddler.
• double aluminum spines: they are really great to give stiffness to the body to resist string tension. This has been my biggest issue with my previous travelele and travel guitar builds.
  Here the one i bought https://www.castorama.fr/plat-aluminium-brut-10-x-2-mm-1-m/3232630508855_CAFR.prd
• 3d printed concealed gear tuners: these work really well, but in order to make them work I finally had to print the gears with 45degree with supports, otherwise they would not keep enough tension (see photos). I did not bother try printing with PLA, and went for PETG, because I heard in many places that PLA should not do good. Note this brings significantly down the manufacturing cost, comparing to using regular gearead tuners (>4EUR), while maintaning reasonable tuning capability (I cannot stand friction tuners on a ukulele).
  I find adding an axle, that made of hanger wire, improves working of the gears when there is little tension from strings. I have also tried putting a nail to keep the drive in place, but I am not sure it is actually needed.
• rounded neck: I think the rounded neck might be more comfortable on the hands, but the printed frets leave some steps... all in all I do not advice.

Surprisingly printing the body front is probably the most difficult part of the body, and mine was far from perfect: ti would probably be better to print with supports.
Regarding the neck, I think it would be better to print a filled version, but the script generate a different shape that is more complex to print.

Here the further things to improve:

• print a filled neck back: the empty neck gives some kind of sound when touching the neck and some "cheap" feeling. I wanted to do that in the first place but the script would generate a different shape for a full body, so I stuck with the "shell" form that is easier to print
• add more adhesion between the body front and back around the gear. In my final build i just added 3 screws around where the spines attach to the back of the body, otherwise the front would start raising under string tension. Not sure this is really needed, but it did not feel right.
• increase build tolerance for spine inlet and gears. I had to file manually quite a bit to make all the components fit.
• +3d printed tuner key that works. I managed to get a reasonable print by printing with 45 degree angle and supports, but the key is way too small for the drive holes.+ [EDIT] I have updated the tuner key with larger width, and it now works, although it still required some filing.
• string action: i have tried 2mm string action, just to revert to the default 3.5mm action, and still probably only half of the fretboard is actually usable. I plan to try 4mm action and keep whatever it comes out of it (the rest of the frets are too small anyways).