I am much much more visible than what you are trying to do. But I wear a reproduction oldschool PLA hat (aka Mao hat), and even a hammer and sickle pin on my work uniform (sometimes also a Marx pin). I had a lot of fun with cloth facemasks (got stuff like antifascist action, left unity, hammer and sickle, YPG, pro-Palestine). Even just wearing the hat, I get a pretty good amount of random leftists making a point to get my attention or outright start talking with me.
All of this really doesn't help you, but it is fun to have those moments and know they know. One that I think would work for you is the anarcho-communist/antifascist/CNT-FAI flag. Most people don't seem to notice that one unless it has the "Antifascist Action" words. I think some folks that aren't aware of it might just think it is some kind of nautical thing. The "Three Arrows" might also work, but it does rub some folks the wrong way (given that one of the arrows originally was against communists). But I personally replace its meaning with capitalism given how it isn't used by any political party in the US and is just seen as another antifascist symbol. Or something ACAB might work, I have seen shirts that have it seem like it stands for "All Cats Are Beautiful".
We most certainly aren't union (though I do wish we were, and I do very openly talk pro-worker/anti-capitalist). I would sign a card the second one were to be presented. I have actually been perma-banned from multiple un-official subreddits that are about the company for saying the "forbidden" word. If we were to try and do it, we would need to have multiple locations also be down. As the main fear is about how they "will shut our store down fast as fuck".
I started with the pins and face-masks during all of the protests in 2020. Just initially figured I'd just keep wearing stuff until someone finally stopped me. But aside from corp banning hats for some reason that literally isn't related to me (I am for real about that too). I only ever got told I couldn't wear my mask that said "Antifa" and another that was left unity (mostly because it had an AK).
It would probably surprise you how I haven't had any customers/clients go out of their way to attack me or even send shit to my bosses (especially given I live in the middle of NC). I even had a guy that I have helped here and there over the past 4 or 5 years and is/was very right-wing be concerned if any of my co-workers have come at me for my openness. He has over time become more apolitical because I have shown him how to better use the internet. Still a very strange guy, but he truly respected how I firmly stand for real-ass working people and that I went waaay above and beyond to work with him when we first met. He was even a big fan of my "Mao hat", and said it looked good on me. lol. I had one older black couple be super jazzed to see it too. The husband was a Vietnam vet (which I was thinking would go badly when I saw his vet hat), and he just kept going on about how great it looked. His wife also said that it made me give off very proud vibes (kind of like how some folks talk about liking a man in uniform).
We also have a wall in the back of my area that we at some point started putting up stickers we had found laying around. I have covered most of it in leftist shit (including some fuck cops type shit just without profanity). After three different direct bosses and a GM, no one even tries to stop me at this point. Though I am sure if a much higher up person were to see me and the wall, it would be a firm ban on my nonsense. Until then, I openly try to answer good-faith questions other folks have. Which seems to really soften their stance on socialism and see how little they actually know about it (outside of the constant shit being said in our media and from politicians).