One solution I've used in the past is to make the heatsink fit the Mag tube tighter so it has to be forced in and will stay where you put it. Then you just don't insert it all the way, leaving it sticking up the extra 1-2 mm you need. The way I've made it tighter is to drive a center punch into the edge of the heatsink at 3 ezually spaced spots around the outside edge of the heatsink. When the punch drives into the aluminum, it slightly raises a dimple around the punch. Most sinks fit pretty closely so it doesn't take much to get them so tight that you have to force in the sink. Just don't drive it too deep because it can be a real pain to get out if it's too tight and gets driven down too low.