I was asking this when I first got my KL3 too.. the old F70 beamshaper might be better if you're concerned about bulk.... I don't have one so I don't know if it fits exactly, but it should since the KL3 is supposed to be a 1.47" bezel.
I diffuse my torch with a 'bottle cap' mod kind of thing, it works fine, the diffused light is decent and usable, but the KL3 is approx rated in the 30 lumens range and if you are going to use 'flood' mode 90% of the time, maybe the new 5W dropin might be better and brighter. I'm waiting for reviews on that.
But the good thing about the tight KL3 beam is the throw and 'blinding' effect it has. With the beamshaper, it will be a very flexible setup. I prob will keep it unless the 5W really puts out helluva lot more light with decent throw.
P/S Yup I believe its FM64 too