John in NM
Just use masking tape. Not blue (or green ) just regular cheap masking tape. Use lots and go far past the void edges. Works great, I worked in a mesquite shop for a couple years and we did a lot of this.

We also used 2 hour epoxy rather than longer cure. Just more time to leak the longer the cure. Large voids you use less hardener as the epoxy is usually exothermic and large masses will go off quicker. The quicker they cure, the more likely you are to get cracks on cooling or even bubbles if the stuff boils (seen both).