That might work, but if you're going to be making a bunch of big things like that in the future, I think I'd get a bigger crockpot.
I did that too, many years ago. It was okay and it worked, but the next day I went out and bought a much bigger crockpot for things like this. I ended up with four crockpots, ideally suited to whatever I was cooking.
Foil's okay but try to keep that space between the lid and the base sealed as much as possible. And check for drying; you might have to turn it during cooking a few times to keep it from drying out.
Adopt a kitty and save nine lives.