¿How can I connect custom domains for the "microwebsites" my clients create?

Hello Indie Hackers family!

I've been a little stuck in this feature my users are requesting me. Users create digital catalogs as websites in @mareaalcalina, but I handle all the hosting with my domain and all the access to the database. Is there a simple way to let my users connect custom domains without granting database access to the custom domain? Do I have to create a new database for each custom domain my users connect?

If any of you have already made this feature for any of your products it would be really helpful to hear some advice.

Thanks!