You could get an alarm with a proximity sensor (also called a radar sensor). True, a lot of them do have plenty of false alarms. But if the installer takes the time to properly adjust the sensitivity, the false alarms will be kept to a minimum.
But in your case, I would get a hood lock and either a backup battery for your alarm or a self powered auxiliary siren. If the wires to the backup siren are cut, it will continue to wail because of its built in battery.
I have installed several hood locks when I was a car alarm installer. I even remember one guy didn't wanna spend the money on a hood lock, so he had some chain and a padlock on his hood. When he popped it, he had just enough room to reach in and unlock it. Maybe that would be an option for you.