I know pre killed ones vaccum seal better becuase when thier bodies are soft they will of course fit perfectly, sometimes you get frozen rats in such wierd and awkward positions, and its hard to make them fit together.You can put more in a bag when they arent in such wierd positions .Its funny, i posted a picture awhile back where the rat looked like it was alive and trying to walk away on all fours. But yeah im certainly going to get one of those soon, it sounds like a god idea.