i included thermal putty in the filters, but it's not the same as actual thermally conductive adhesives.
i suppose in a pinch, you could try zinc oxide powder mixed into the resin component of epoxy, ZnO is what's used in silicone heat sink grease. or aluminum oxide which is used in those thick white heat sink insulators sometimes seen in switching supplies. the only electrically insulating oxide that's more thermally conductive than aluminum oxide is beryllium oxide, but you don't want to mess with that in powdered form, it's a health hazard.
I also have used Artic Silver. It bonds strongly. Thermally, it works very well but it is expensive. It also has a limited shelf life once that it has been opened.
you can only tell if it's conductive, you need a "megger" to find out what the dielectric strength is. that's important to know if you are applying more than 12 to 50 volts to devices on a heat sink.
To further illustrate what unclejed mentions, an insulator may posses a very high initial resistance, but could suffer from dielectric breakdown at higher voltages.
To measure the dielectric strength, you require either a Megger or a Hipot tester.
You REALLY need to know the voltage level you are attempting to insulate from.