Hmm, I haven't heard anything good about the Thunder Reds. I reallllly didn't get along with the Pahoehoes. I really want to make a 5 string happen... I've got a nice big chunk of pine to carve a body from, and I think I could really use anything for the neck. Fretless, 5 or 6 string, piezo, perhaps DIY my own buffer and a fixed HPF?
Did you mean "LPF"?
I had to go to a 25.5" scale before I got a low B that worked for me, no matter what strings I used. At that scale, the standard EADG gauges work when tuned down a fourth, so no special string is required for the low B (other than to make sure it's long enough!). I've had good luck doing this with the usual choices of urethane/nylon strings and the wound Savarez 640R.
I realize that makes the instrument much bigger than a uke, so it might not be an option for you. On the other hand, there are plenty of necks already available, which saves much effort.
Since you are interested in "rolling your own", you may find some useful ideas here:
http://www.talkbass.com/threads/sub-short-scale-talk.644956/