If using Netlify, it's possible to setup a reverse proxy using your .toml file to establish a subdirectory on Ghost(Pro).
For this configuration to work successfully, you must be on a Business plan with a subdirectory enabled. You must also paste the exact configuration below into your netlify.toml file, updating the values to match your ghost.io subdomain and custom domain, so that it meets our reverse proxy rules.
Note: Ensure that your setup references the subdirectory path you are wanting to use (e.g. /blog/).
[[redirects]]
from = "/blog/*"
to = "https://<subdomain>.ghost.io/blog/:splat"
status = 200
force = true
headers = {X-Forwarded-Host = "<yourdomain.com>"}
[[redirects]]
from = "/blog"
to = "https://<subdomain>.ghost.io/blog/"
status = 200
force = true
headers = {X-Forwarded-Host = "<yourdomain.com>"}