I'd feel guilty for not replying to this...
Keep them as simple as possible, telescoping nunchucks are awkward to work with and weighted/balenced strangely. In addition you cannot thrust with them. Nunchucks where the top ends connect together by screwing one end into the other are annoying, I tested a pair- the remainder of the nunchuck chain just rattles on the inside and it seems abit bigger than I would expect a self defence stick to be (escrima stick? I haven't trained in stick fighting so correct me if I am wrong). A 24-28 inch stick with a rattle in my trousers just doesn't seem very practical.
The recoiling would be a very large concern if the intended use is self defence, as catches/rebound and control needs to be practiced -probably alot easier to use another weapon.
Usually (as was the case when I was shopping) they are aimed at enthusiasts/collectors and the materials, bearings, chain length and the sticks probably will not be up to standard. Rubber is quite hard (I had the impression it would be quite soft when starting out), while foam is never a bad choice for a beginner (especially as price is not the concern with home made 'chucks).
Lastly, shorter cords provide higher speed and i *think* more control, whilst longer chains give the weapon a sagging whip like feel, with more power but with much slower movement (depending on your style this may end up unpredictable). Gripping closer to the chain also gives more control and speed (depending on wrist movement) whilst moving further away increases force.
Hope this helps, sorry if it seems like an old bump.