Uhh... I can't imagine how you would possibly scratch the neck... It can bump the frets if you don't pay attention but I have no concern about that.
Also, I have no experience with 3D printing but there's actually a fair bit of tension on the device. I'm not sure something you print will be strong enough.
In the US I can get one for $13... What's the cost for you?
This is the review saying that you can dent your fretboard if you are not careful with the Stretcha. Maybe he is a brute
The tensile strength, density and strength to weight of epoxy (Stretcha) and ABS (3D printer material) is not very different. I don't think that it would break.
The cheapest one I've found that ships to Thailand (from Ebay Japan) is $40.97 inc shiping. I'll start with the design during the spare time at the office. There is no rush, just for fun. In the meantime, I'll keep doing like @Joe Bfstplk but without breaking the strings