I usually put it in my cargo pocket for easy access since its hard to access the hip pockets in the saddle. I picked up a HME screw in cup holder off eBay recently and I like using it for longer sits.
https://www.ebay.com/itm/HME-Big-Ga...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
https://www.ebay.com/itm/HME-Big-Ga...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649