I've done a triple & a quad cutter into lumi halogen cans & very pleased with the results. Been running the 3 LED one for nearly 2 years now.
Used a plumbing 28mm copper end feed stop end for a heatsink. Cut the sides of the stop end to be "fins" that splay out very slightly to get maximum contact area against the side of the lumi can, & with necessary slots for clearance around the bolt etc. Heatsink compound between the fins & the case. Fixed the back of the LED board to the face of the end stop with some artic silver adhesive.
Tape up the control board & slot into the gap behind the end stop perpendicular to the LED board. Not had any problems with overheating, running at (iirc) 75C thermal shutdown. The end stop arrangement seems to conduct the heat to the case quite nicely.
I run both at 75% current rating. I can't say i detect that much less brightness for a lot of extra battery drain & noticeable extra heat generation compared to the 100% current setting.
With the triple with a narrow beam diffuser on my helmet & the quad with a middle-ish beam diffuser on the bars, they're overkill for the type of riding i do 🙂