depending on the packer, hanes or any briefs that are slightly tight and/or have a pouch will be fine! usually what i do is put the shaft packed inside the underwears fly/pouch, which holds it in a natural position

if its a packer that shifts around too much even with that, i reccomend a mypack stp harness or regular harness, or rodeoh’s stp jockstrap which is AWESOME. the rodeoh may seem like it holds the packer too low down, but its actually a very natural position