I recieved a couple of them today and tried them out on various motors to see what they fit. One thing I noticed was it is basically a shroud and relies on the cans cooling fins for output venting. So smooth can motors may not vent as well, or I guess you could drill some holes, but I think the logic was to blow air around as much of the can as possible before venting.
Pictures:

Fits perfectly on a Neu-Castle

Fits on a finned Neu 1515 perfectly as well

And a finned Neu 1512 (same diameter)

Doesn't fit on a 1515 Tekno-Neu square can - Duh

Fits pretty good on a Tekin T8

Fits ok on a smooth can Neu 1509 (a little loose)

Worked ok on a Feigao

Too big for a Castle CMS36 can

Way too big for a 540 sized can

And the only output venting is via the ribs on the can.