Let's see what your best try looks like. (See boilerplate on Why We Make You Do It Yourself*) Don't forget the preceding rule that issues an explicit redirect-- not rewrite-- to people who ask for the ?querystring version by name.
* I can't re-post it too often or this forum will itself start falling prey to Duplicate Content ;)
You should have one rule that redirects request with slash to new URL without. This fixes the Duplicate Content problem that you wanted to create for your site. This Rule will also have a preceding Condition that checks THE_REQUEST.
Your site should link to URLs that do not end with a slash.
The second rule is the rewrite and is just one line of code.
As the most asked question here, there's a couple of thousand previous threads with the code. The last time we went through this all was just a few days ago.