Size-wise, I made this 3-stringer, “Blues Cat,” between that of a ¾ and a 4/4 double bass. EAD tuning and a flat fingerboard. There are several sites with dimensions, etc.
And used "Slap Happy's" weed wackers strings:
Nice thumpy, definitely upright bass/not electric bass sound (tho' acoustically, not terribly loud). Since these pics, I've added a piezo under the bridge (as well as a couple of other corrections/mods.
Tom, I used a rod piezo. No recording sample, but with the weed wacker strings and only 1/8” thick plywood over its frame, it does give a decent replication of a big bass' sound, IMO. The thing is flat- front, back and sides. (The project began as toe-pincher coffin case, but before long, I decided that what I really needed was a bull fiddle that could double as a raft in case of a flood or something. (Always thinking! :D)
Incidentally, I did angle its neck, tho' not as severe an angle as has a real, professionally-made upright bass. Incidentally, the picture on the left is the back of Blues Cat. I put the direction, “Up,” so the bass player would know its orientation without having to turn it around. ^^
I did a 2 string fretless bass using a Fender Jazz Bass strings, it is 32" scale length. Used a fairly big cigar box, built like a neck thru cbg except the "thru" part keeps going to the floor. It has a nice bass sound but not a lot of volume. The stick is as crooked as it appears, it was a piece of scrap hickory from making a bow.
I'm building one right now. Cigarbox-style from a wheelbarrow: