Frequently (if it's been a while), it packs fairly nice. Be very careful removing the "plug" of lath. It will sit there tenuously but if you touch it then it comes cascading out.

Spray foam is going to expand everywhere and make everything difficult. Remember you still have to pull the wire into this cavity and get the box to fit in there.

I might try (using a Fein or similar cast cutter or a rotozip) cutting a horizontal slot and just jamming a piece of thin rigid material (cardboard or plastic) as the damn) and fix up the finished surface later.

Thanks for the tip. I used a Rotozip and made a dam out of aluminum flashing. Not much spillage. Thanks again.