I would be leery of normal epoxy at that temp , most are only rated for 350-400 F , and are not the worlds best heat conductors .
For some reason JB weld is rated to 550 , and since it is metal filled " should " conduct heat better .
Any reason you can't use an IR thermometer ? Granted it would not be as accurate , but much easier .
When we had to take accurate temp readings of metal ( non molten ) , we would normally try for a solid mechanical connection of some kind . Bolted on , in a drilled well , soldered on ,etc .