Thanks to user fortminor from DigitalOcean community thread, I finally figured out how to add LetsEncrypt to basic ServerPilot installation. Lets walk through the step if you have DO droplet configured with ServerPilot.

First, run below script as su/root.

$ cd /usr/local/bin

$ wget https://raw.githubusercontent.com/lesaff/serverpilot-letsencrypt/master/sple.sh

$ chmod +x sple.sh

$ ./sple.sh

Above script will get the ServerPilot LetsEncrypt from GitHub.

After completing the wizard, add the autorenewal to crontab.

$ crontab -e

Insert below line to the crontab & save it.

0 */12 * * * letsencrypt renew && service nginx-sp reload

Then, navigate to nginx-sp vhost.d folder.

$ cd /etc/nginx-sp/vhosts.d

Edit your app's ssl.conf & add below settings at top of the file. This to redirect http requests to https. Replace example.com with your domain.

server {
   listen 80;
   listen [::]:80;
   server_name example.com www.example.com;
   return 301 https://example.com$request_uri;
}

Backup your app's conf & copy ssl.conf to .conf. This is to have a backup in case ServerPilot reset your app's conf file.

$ mv example-app.conf example-app.conf.bak && cp example-app.ssh.conf example-app.conf

That's it! Now restart nginx-sp service for the finishing move.

$ service nginx-sp restart

Here is the sample image used on this website.