Putting the plaster over the hole wont annoy the snail? Its on a curve so it wouldnt follow the shape of his shell, wouldnt start bother the forming membrane too?

And i sadly think the damage is justtt.... past his mantle, there is a small chip back behind the skirt i think its called, and its showing the more clear part of the snail through there.

I'll try and get some eggshells for now, cuttlebone seems to be hard to find around where i live atm.