Yes that true , it made it a lot easier with certificate and applying the  certs for multiple objects.
 
Ken Felix
 

sounds cool....can I get a copy of what you have so far?

 
1: upload the  new cert and then change the useage to the new cert 
 
2: delete the old certificate
 

Here is the github-repo for the current version,  it's still a bit early but it does work.  The --help feature should give you all the parameters required.  I'm trying to write it to have a config file as well so that you don't need to write the whole command every time, the beginning framework is there for this in the script but is not fully tested yet so I suggest using the cli commands.  Also this works with either password or API-Key authentication.  Let me know if things aren't working for you.
https://github.com/absmith82/fortitools

I agree that would be ideal, however, that isn't currently the case, and as far as I can tell fortios doesn't allow you to access the python shell/ecosystem so in the meantime we have to work with the tools they allow us to.

If anyone is still looking or in need for the Letsencrypt solution for fortigate here is a working process ( was testing it for the past few weeks )
 
A linux VM ( CentOS 7 or 8 ) for the certbot installation and the scripts downloaded from here ( https://github.com/gdoornenbal/dehydrated-certificate-installers ) not my repository, just sharing some great work.
 
also if you have a hosting that is usping Cpanel, you can download this plugin so you don't need to add/remove TXT entry every time you need to issue or renew a certificate ( https://github.com/badjware/certbot-dns-cpanel )
 
there are also plugins for cloudflare ( https://github.com/certbot/certbot/tree/master/certbot-dns-cloudflare )
 
as a side note, you can split the shell script in 2 parts, one for the HTTP acces the other for the VPN, if you do so, you will need to edit this part of the script:
 
set livecertdate [exec echo | openssl s_client -showcerts -connect $host:$sslport 2>/dev/null | openssl x509 -noout -enddate | cut -d = -f 2 ]
 
into this
 
set livecertdate [exec echo | openssl s_client -showcerts -connect $host:$sslport 2>/dev/null | openssl x509 -enddate -nooout -in cert.pem | cut -d = -f 2 ]
 
when everything is ready, just setup the cronjob for the renews and you're done.
 
as a bonus and extra precaution you can install the cockpit package for web access to the VM and a 2FA via google authentificator whick you can setup for both SSH and cockpit access.
 
if someone needs help or a better clarification, just drop me a PM

I wonder if the Fortigate could use an Automation/Stitch to self-renew the Lets Encrypt cert every 50 days or so... 

It looks like Stitch/Automation supports AWS Lamba or Azure Functions as actions with time based trigger .. so if I read that right then could have a Stitch call an Azure Function every X days that renews the cert file, then validates the file. 
What I don't know is if the Azure Function or AWS Lamba coud return the cert through directly or if it would have to place the cert file in a location.
 
Optimally, once the Stitch that calls the AzFunction/AWSLamba finishes another "Action" would run that runs a Cli_Script on  the fortigate that would then import the renewed Let's encrypt cert, then update the firewall SSL cert + SSLVPN cert + etc  AND would log success/failure into the system event logs 
 
// Edit ..it looks like you would need to use the generic Webhook action to get results if your goal was to have the firewall "pull" everything  (vs. having the AzFunction/AWSLamba "push" into your firewall from the public cloud using firewall API calls)
 
So far the real problem I see is that the info on "Chaining Stitches" is missing, the section on it only talks about delays.
 
https://docs.fortinet.com/document/fortigate/latest/administration-guide/137181/chaining-and-delaying-actions
 
// Edit 
BTW - here are some quick results for starting points on AzFunction or AWSLambda
GitHub - MarcStan/lets-encrypt-azure: Azure function based Let's Encrypt automation for Azure CDN & app services
Let's Encrypt SSL Certificate to Azure Functions - Microsoft Tech Community
 
Free SSL certificates with Certbot in AWS Lambda - Vittorio Nardone
 
 

V7.0 already supports "Let's Encrypt", you can try it.
https://docs.fortinet.com/document/fortigate/7.0.0/new-features/822087/acme-certificate-support