I don't know if it would fit as I don't have a Fenix headband to test out with my TK20 but I think that the TK20 is too heavy and big for headband use. It would give your neck muscles a real workout though, which could be seen as a benefit if you're one of those guys that wants a neck thicker than your head.
It doesn't fit and it would be a little side-heavy if it did.
If you insist on a neutral LED light to fit in there, regular and tactical Quarks fit, the mini is too small and I don't have a Quark Turbo to try, but it probably doesn't since the other quarks fit at maximum capacity (held by 3-4 turns)
The HP10 can be combined with the Fenix headband so you end up with 3 good lights on your head.
Actually it does fit and you dont need to remove the rubber grip. It's not a perfect fit and you do have to place the clamp towards the rear end of the light. You can get a few turns in of the screw to secure it.
It is not the best solution at all as you can imagine. The torch will slowly creep down on one side as it's so heavy, also tips forward as you cant balance it in the middle. Maybe two TK20's, either side of the head would work as a counter balance.
The niteize head band works much better but that has the same problem of drooping down one side. It ends up being propped up by your ear.
A TK20 is a VERY throwy light and wouldn't be much good on a headband. You'd end up blinding yourself with the blowback even on the low setting. Consider something smaller and more floody like a Quark AA or Nitecore.