I've used Scotch DP 270 epoxy with good results.Remove the heatsink and mounting board,,coat both sides of the esc well and scrape the still wet epoxy off the tops of the mosfets so the heatsink will have intimate contact with them.
You can buy it here
http://www.offshoreelectrics.com/pro...prod=ose-dp270 and elsewhere (google "dp 270 epoxy" )
HTH
Steve