Umm a velocity factor of .50 might be attainable with a variation on
leaky coax... Some of the cheap stuff is already as slow as .66...
Off the top of my pointy head braided wires wrapped on your form (same
as you are currently doing) then potted in an appropriate dielectric
material for filling the braid interstices should slow things down
some more... The question extant is what material for potting (or for
adding to your potting resin) will further slow progression of the
wave front along the braid...
We know that solid polyethylene dielectric on coax results in a lower
vf than the foamed materials.. So it appears that denser material
results in slower wave progression... Perhaps UHMW, or similar might
suffice...
Other thoughts come along, such as a flat ribbon with a deeply
embossed pattern then filled with dielectric might be slower than
braided wire... Or even a ribbon of screening material similarily
filled...
Dunno... Interesting problem...

denny / k8do