Many regulary gel pens will work just fine and roll smoothly over the fabric. Try a couple. set the ink with your iron (I sandwich between paper towels) and then throw your test scrap into a cup of hot water and let it sit an hour or so. That will let you know what pen will be permanent or might run.
When I write on fabric, I back it with several layers of masking tape to stablize. Works great and easier than ironning freezer paper on it.
Good luck