https://www.amazon.com/dp/B01M0FJIYF/ref=cm_sw_r_cp_apa_i_cZmbFb4V28N9N

Something like this, but put it on your shoulder strap and let it hang in the front if you want it easy to get to. Any of the bottle holders are a pain without swinging the ruck onto one strap.

Bladders are great, if you get one, just throw it in the freezer after rinsing a draining between rucks, keeps bad tastes out.