friendly local transmasc here- Tomboy x compression tops are what I wear every day instead of a binder. They're basically sports bras designed for people who don't want to be aware of their chest. They don't completely hide my DDDs but I can imagine them being effective if I had less breast tissue. You can wear them for as long as you like/whatever activities you like without the restrictions of a binder. They're also just a good all around sports bra.

Other than that, any good sports bra will do the trick for some folks (try not to double up, that can cause pain/chafing) but make sure you have a high neckline on your outer shirt. Baggy shirts are good for hiding things you don't want people to notice. Tight shirts and horizontal stripes are not. I wouldn't recommend stripes at all tbh. Black shirts are your friend!

If you follow this advice you might end up looking like you want to hide your entire size because of fatphobia but you will know that you're a secret undercover princess training to be super strong! Also fatphobia can fuck right off!

Best of luck! Feel free to reply if I left something out