Originally Posted by
Cordite
Charcoal should do it and should add rigidity. A problem with pure epoxy can be is creep, especially at higher temperatures.
The ancient Neanderthalers used an advanced mixture of birch bark resin (extracted by heating birch bark in a closed container, i.e. no air to it) with ground charcoal. The charcoal resulted in a more rigid, non-flowing, compound about as strong as epoxy once it cooled. They used it to attach spear and arrowheads and flint blades into knife handles.