I used a 1" piece of pvc pipe to fit a 1" aspheric lens on my rexlight. its removable and focusable with 1 hand. to give it a tight fit i added some velcro tape to the inside of the pvc pipe (the fuzzy side of the velcro).
it works like a charm, and i think an lod would be even better with the same aspheric, since it has a wider beam angle. the reflector on my rexlight truncates the beam before it hits the optic, where an lod reflector lets more of the direct beam hit the optic. light coming into the optic from the reflector is useless. the problem is you need a large optic to capture the whole beam.
Hmm or a small one that has a short focus length
If the angle of the beam is 70
the lens only has to be 0,7xfl x2
if the beam is 90
the lens only has to be 1xfl x2
If the angle of the beam is 120
the lens only has to be 1,732xfl x2
If a cree has a beam angle of 70
and the lens has to be ( just a guess )12mm to fit
the fl has to be 8,571mm ,,or shorter
then you would capture all light coming out of the cree