Securing the car / upgrading alarm system

With the common knowledge that our cars are easy to steal and havig heard of some going missing not too far from my area, I'm becoming increasingly worried about my car's safety.



I found out that it'll cost over £600 to get a Viper or similar system fitted so just thought I'd ask on here to see if anyone has any advice to make the car more secure without having to fork out £600+. Quick little changes like just relocating the OBD port, or a system that can be fitted for less money.



Any tips or advice would be helpful



Thanks!