Yep thats it. The canadians use a "panic snap" off a lead rope as their release. The vicegrip from the American disign is better in my opinion.
The panic snap might be harder to rig initialy, but likely more resistant to corrosion than the vicegrips would be. The vicegrips would be more easily adjustable as for trigger pressure than the panic snap though.
Did you use the same roof door setup, or rig a guillotine type door? What did you do as for killing the cat, small hole to shoot through, or lift the door a crack and shoot?
Seems interesting enough.