You cant do it with squarespace directly.(or Wix, or other framework based website service providers) as they dont allow for custom files at all paths.

If you're certain to stay with Squarespace, you can front the site with a service like Cloudflare. Use Cloudflare as an intercept to your squarespace website and perform a URL rewrite for the .well-known/nostr.json resource, along with response headers for CORS.