I can't speak to an alarm, but I couldn't bring myself to trust a steel cable to keep my kayak safe. They can be cut with a pocket knife or a small pair of pliers.
I went to HD and bought a chain and some electrical cable flexible tubing to keep the chain from scratching my car. I run this through a scupper hole and through my front and rear windows, around the frame of my car. Padlock and its as safe as I want it for a short while.
Quick trips into the store and I trust it, anything longer it wouldn't take someone much to snatch it right off the roof of my car.
If it were me, and I would just stay in a MOtel and bring it into the room with me.