(I'm also a trans guy planning on being fully out when I go to uni)
I wouldn't make a shirt saying that you're a guy on it, personally. I feel like people will use it as an excuse to ask you awkward questions and stuff. Just act like you're a guy and you expect everyone else to believe it, that's what I do! Eg use the guy's toilets. Also correct everyone if they get your gender wrong, even though it's really scary to do that in my experience.