For a quick solution until you find a cupholder you'd like to install, just pull the leg bolster on the passenger seat forward and use the opening to hold your cup in place. Of course if you have a passenger along you won't be able to use that method ... but they can hold your cup for you.

Personally I just use a 90 oz SuperMegaBigGulp cup I got at Sheetz. The bottom is wide enough that it sits on the floor next to my left foot without spilling ... usually.