Force sites to live underneath a slug

So, I have am setting up a subfolder multisite install under a top level domain. There will be a handful of subdomains that only superadmins will create, and we will use domain mapping to point those to the right place. The rest of the child sites will be user created, similar to wordpress.com

My question: can I force all the user created child sites to live under a specific directory?

So, I have:
http://www.example.com/ – Landing Page for primary site
http://www.example.com/academy – a page for the primary site
http://www.example.com/sites – A page that has all the blah blah for creating your own site
http://www.example.com/sites/usersite1
http://www.example.com/sites/usersite2 — User created sites that live under the /sites/ slug.

I know I can use a MultiNetwork plugin, but I feel like that’s adding an awful lot of complexity for a relatively simple task.

Any thoughts?

Thanks