you will probably not believe me when I tell you, but normal RTV sealant, has rather good thermal properties. way good enough for 5W that's for sure... maybe not what you would want to use on an AMD cpu.... but for keeping a block on a chipset, without any damage to any component what so ever, and completely removable afterwards... you will be hard pressed to beat a dab of sealant.... excluding of coarse when you do have the mounting holes.....
be careful though... sometimes the stick, is so good that you might not be able to get the block off again.... I usually put a couple of dots off silicone thermal paste (or as3 if you wish) in the middle, then squeeze a neat thin little circle around it, with about 5mm radius from the dot of paste, to give it somwhere to squeeze into. carefull not to complete the circle, as you would want to give the trapped air a path to escape, then just neatly press down on the block, making sure is squeezes into a nice firn contact, and let stand for a while to give the RTV some chance to dry.... [UTTAOR]