Pack your stuff in ammo cans. The 20mm ones are pretty roomy. You might bolt them to the floor in high crime areas but sometimes just covering with trash provides cover. Old fast food bags and cup, all carefully cleared of anything that might rot or smell, pilled up for artistic effect can make gear so theft-proof that you could leave the windows down. Also helps if the vehicle is a bit of a beater. Keep the operating parts in good condition but let the outside and interior go a bit. Let mud and dust and dings ride.