Only non-custom-made, short-scale flatwound 090 I'm aware of is made by
Rotosound,* the low E from their shortie Jazz Bass 77 set (
RS77S):
Rotosound RS77S Jazz Bass 77 Monel Flatwound Bass Guitar Strings - Short Scale
The D string gauge in particular is significantly lighter than what's in the Ernie Ball 2837 set (050 vs. 054), the rest is in the ballpark.
To the above you may add two
D'Addario Chrome guitar flatwound strings, a 020 and a 030 (buy a couple more to qualify for free shipping):
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Yes, they will be long enough (about 38" winding length, which is plenty for your Longhorn); yes, though flats they may not have the same tone as the Rotos but, isn't that the case for plain vs. wound strings on standard guitar?
So far we're
under fifty bucks shipped, which is not bad.
Me, I plan to get an überlight set of all D'Addario guitar Chromes (plus one plain high E) for my VI-type instrument in the near future. Their largest string is a 080, which I would use as a low E. It should make for an interesting challenge...
* [I don't really recommend said low E based on my personal experience: very floppy and stiff at the same time, so 1) very jumpy pitch if you don't pluck/pick supersoftly, and 2) you may run out of intonation at the bridge.]