Dealing with deep voids while repointing

Repointing my old stone foundation and some voids are quite deep, 8+ inches. I've tried just straight packing them with mortar, which works but consumes a considerable amount of mortar and time especially in cases where the void opens up the deeper you go. I've also tried wedging a stone or two while packing with mortar but it proves tricky to get the right sized stone.

Is there any methods that you can recommend?