VPC NAT Gateway の利用費が毎月 USD 50 ドルくらい掛かってワロエないのでメモ。
t2-nano の Ubuntu で NAT Instance を作って節約する。

# Enable IP forwarding
sudo sysctl -w net.ipv4.ip_forward=1

# Make IP forwarding permanent
echo "net.ipv4.ip_forward=1" | sudo tee -a /etc/sysctl.conf

# Configure iptables for NAT
# Replace 'ens5' with your network interface name
sudo iptables -t nat -A POSTROUTING -o ens5 -j MASQUERADE
sudo iptables -A FORWARD -i ens5 -o ens5 -m state --state RELATED,ESTABLISHED -j ACCEPT
sudo iptables -A FORWARD -i ens5 -o ens5 -j ACCEPT

# Save iptables rules to persist after reboot
sudo apt-get update
sudo apt-get install -y iptables-persistent
# During installation, select 'Yes' to save current rules

# Or manually save rules
sudo netfilter-persistent save

最後に下記の公式ドキュメントに記載の Disable source/destination checks と、Update the route table を AWS コンソール上で実行すれば OK.
 
NAT instances – Amazon Virtual Private Cloud