I would like to create my own iptables rules from scratch instead of using the YaST > Firewall custom rules.

If I create my own firewall script with those rules and disable the built-in firewall service of openSUSE - where should I put this script in order the system to run it at proper time during boot? I would like to do it in a clean way so that if I change my mind later I can still go back to the built-in firewall.

P.S. My main idea is to drop all packets and to allow only what I really need. Does that imply any worse security than the default firewall scheme?