redirection – Drupal multisite with subdomains: how to force /web in the URL?

I have installed a Drupal 8 multisite using Composer (as described by kalose ): basicly the folders created as subdomain-folders have been replaced with a symlink to the resp. subdomainfolders in /sites.
I have 2 websites on 2 subdomains, with following URL’s: sub1.example.be/web and sub2.example.be/web

I placed a index.php in public_html with a header ("Location: https://sub1.example.be/web"); command to have all example.be linking to the sub1-site. Works fine.

Question
Where can I put a similar index.php to help users that use the old link “sub2.example.be” to redirect to “sub2.example.be/web” ?
Or any other solution to help users that forgot about the /web-suffix in the new URL.