[Some of what follows has already been touched upon, and is in that case meant as a summary rather than lack of recognition.]
There are essentially two problems with guitar scale bass strings.
1- Gauges, for which there are two approaches
--- increase gauges up to those typically used for the lowest four of standard 5-string basses. In practice, their low B becomes your low E. This gives you normal bass tension. On the other hand, the lowest string(s) will have less clarity and more of a chorusy component in the tone, especially up the neck;
--- use normal gauges, for less tension, and often keeping to the lighter side of the spectrum (say, 40-95 max). This restores clarity but yields a different playing experience, with a lot less tension. It's not just a matter of it meeting one's preference: the plucking hand touch needs to be controlled in order for the strings not to smash on the frets, and also for the pitch envelope upon the attack - which normally goes sharp, then flat, then Goldi - to be reined in.
Most guitar-scale acoustic basses go a step further and use nylon-core strings, which are more flexible and pitch-stable: they still require a light touch not to flop around, though. These are non-magnetic (silver-plated copper winding on a nylon core) and strictly for piezo pickups; however, there are now
- nylon-core flatwounds that work with magnetic pickups thanks to their metal winding
(Galli UXB910C aka Invalid Link Removed, 23" scale version, 26.5" winding length, gauges 60-80-100-130;
[Invalid or Expired Link Removed], 25.6" winding, gauges 41-54-75-100;
note that the above does NOT apply to the new Aquila Flat Spirals, which are copper flatwound on a synthetic core, and are therefore non-magnetic; also note the aforementioned two approaches both represented here, with one set having fat gauges to compensate for the reduced scale, the other sporting normal gauges), or
- metal-on-metal strings with very thin cores
(Invalid Link Removed, roundwound, and JC106TB, flatwound: gauges 45-105, made for the Tiny Boy electric mini-bass), or
- others with extra-flexible braided/stranded/rope-cores (also Dogal; unfortunately only available in long scale - the Jonas Hellborg signature sets, Invalid Link Removed roundwound and Invalid Link Removed flatwound - and uke-bass scale - UBS162, made for the Big Island electric ukulele bass).
2- Fitment. Past a certain gauge (080 in my experience), strings can break when directly wound upon the tuning post, because the stiffer they are, the less they like sharp bends. The risk is somewhat reduced the more inherently flexible a string is (roundwound, thin core), the larger the tuning post is, and the lesser the tension (ensured by lower gauge, shorter scale or both), but it's there. Ideally, to avoid the above you need at least the low E (and A, if it's fat) to taper between nut and post. However, since guitar-scale basses aren't common (except parlor-size acoustics, as mentioned above, but most of them use nylon-core strings, as also mentioned), there aren't many sets that fit them correctly.
Two that do are the [Invalid or Expired Link Removed] (roundwound, 65-125; made for the original Fernandes Pie-Zo bass, aka Nomad Bass), and the La Bella MBLS (aka MBSLS on the eBay; made for the Gold Tone MicroBass, flatwound, 49-109, 28" winding).
Alternatives are
--- manually unwinding them to length (better if secured with solder or heat-shrink tubing so that they don't get unwound further);
--- with the thin-gauge approach, as outlined above (a max 095 low E), one could try and maybe get away with having the unmodified string wound at full gauge on to the post;
--- ordering custom-length strings from manufacturing companies that offer such a service: Newtone Strings in the UK, Pyramid in Germany (the only one that makes custom flats and nylon tapewounds, besides rounds). Both ship worldwide;
--- modifying the instrument to accept more standard-length bass strings. In your case, this would entail replacing the trapeze tailpiece with a bass 4-string stopbar positioned at the butt of the body, even at 45° right on the body rounding, next to the strap button: if the new length from there to the E-string tuning post amounted to 32.5, this would allow you to use commonly-available short-scale strings, 32" from ball-end to start of taper. From that point on, you'll be able to decide if you prefer light, heavy or somewhere in between.
Or, you could add string-through-body ferrules, also at an appropriately far position.
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Do you have them? I've been wanting to confirm the winding length of the lowest strings (from ball-end to where they taper) for AGES. My understanding is they're around 31", which is too long for the OP's bass. However, if you have them at hand and can measure them, it would be greatly appreciated!
I may still have a set, I'll look.
