How to prevent Epoxy from sagging/dripping
Microballoons. Tiny glass or phenolic bubbles. They are the standard product for mixing into epoxy when you need a paste. Should be available at any place that caters to boat repair.

Plugging the hole with masking tape or similar may also work perfectly well in this case. If you can arrange to hold the bolt in with pressure, you could also use a large washer and a nut to hold a rubber or felt washer/gasket against the wall to keep the epoxy in place until it cures.