I’d get one of the Kydex belt holders or a Wilderness Tactical. One or two of these should suffice. You could carry spare mags in your cargo pockets or back pockets if the instructor is really into spraying rounds.
I’ve always gotten away with a photographer/Adventure vest. I don’t get all tac’d out since it’s not realistic for me to have that stuff on when there’s bump in the night… so I just use the photog vest for simplicity.
Go to the “pouch comparison” link in my sig and follow the link to the spreadsheet. At the bottom of the spreadsheet you’ll see a tab for “Rifle - Filtered”. Click that tab and then scroll right until you see the “Belt Attach” column. Set that to “Yes” and you’ll see a selection of belt-compatible pouches.
I here what you are saying about not wanting to wear vest/gear, I too am taking a carbine course this summer with EAG, they require you carry 3-4 mags on your person in some way meaning they don’t require a rig. I am just a regular earth person and nothing special. The last thing I want to do is look like some poser or airsoft guy. But, I decided to pick up a rig, I will be running an ATS m4 modular and my friend will be running a US Grunt Gear m4 tripple light. I talked to a guy that took the class a few years ago and he said he didn’t come with a rig and ended up buying one after the first day, he said it made the class much more enjoyable.