so, if you have the cheap ebay magicshine /clone/ lamp (NOT the 'real' magicshine), and you get the actionLED spreader lens, as action tells you, the lens on the clone lamp is thicker than the spreader lens. I hit up the hardware store and poked around at o-rings. I found a 1-9/16ths" OD 3/32" thick O-ring is the right outside diameter exactly, but its about 1/32nd too thick. I think the perfect o-ring to replace the one on the clone lamp would be 1-9/16ths OD and 1/16th thick, or 39mm OD and 1.5mm thick. o-rings are often sold by their ID, so that would be 1-7/16" or 36mm.
your mileage may vary. the spreader lens looks awesome in my livingroom, the beam is nice and wide now.
Experience is what you get when you don't get what you want.