Found it.
First run the iptables entry
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -j SNAT --to xxx.xx.xx.xx
Then run sudo apt-get install iptables-persistent, and follow the prompts. When it asks to save the current rules, hit "Yes" at both prompts. Now, on reboots, your iptables rules will be restored.
All done, working fine! 🙂