Need .htaccess help removing trailing slash from subfolders (Core PHP Project)

Hi there. I have a project where we are trying to put copies of sites inside subfolders off the root domain.

Like:

maindomain.com/subsite1/
maindomain.com/subsite2/
maindomain.com/subsite3/

All the subsites have trailing slashes. I want to remove those. We do some custom things in our index.php and controller that loads the router (ie. page urls), so I'm not sure if I should be modifying that stuff or the .htaccess file.

I need someone who can help me fix this real quick. I'm sure it's something that someone more experienced than me can probably fix in an hour or less.

Thanks!