Most trans men or afab (assigned female at birth) nonbinary people who want to create a bulge will usually use a packer (a silicone prosthetic) or a sock. There are many different kinds of prosthetic packers, though most trans guys will start with the Mr Limpy as it's one of the cheaper ones. If you're questioning or you don't want to spend money, a lot of trans guys will also use a balled-up sock.
can't say I know from experience, but I'd imagine anything works so long as its comfortable and you're able to keep it in one place without it shuffling around and falling out one pant-leg
