How to carve a mask?

I am working on a fursuit base and I'm now trying to carve out the inside so I can print it and it can actually be worn. Now I tried the shell method but I accidentally did a Boolean for the eye sockets (not visible in the images,sorry) earlier and it messes with the shell by leaving those sockets intact.

Is there any way to close up the eyes again and fix it all? Any other way to hollow the base? Also my app is prone to crashing when I do a voxel of 200 which is the resolution someone recommended to keep the headbase clean for printing, so I dont know what's causing the crashing. (It doesn't always crash, but it has happened enough that it starts worrying me).

Any help appreciated, thank you!