Or put the domain on cloudflare. My site is hosted on github too, with a CF cert. Works fine with https :)
From what I understand (and I admit, my knowledge is pretty lacking around what cloudflare offers) the free part is only the security between the client and cloudflare (and not the security between cloudflare and your server).
On one hand it's better than nothing as I assume most MITMs are on local wifis and such but on the other it gives users a false sense of security.
Then again none of this is of actual concern for node.green
The free plan also supports securing the connection between CloudFlare and your server.
I made my blog hosted on Github HTTPS with this, mainly so that I can experiment some features like Service Workers.