Get an insulated Camelbak, stuffed behind a seat out of the sun they hold ice for hours. Fill 3/4 full with ice cubes and then top with water or maybe a sports drink (if so wash out immediately after flight). Carry a lot of fluids, plan for worse case landout. Put extra water in your crew car or trailer.

Worry about what is important. If you are worrying about cup holders I am concerned what else might be going astray. Especially if you have little XC experience find a good local mentor who can help you set up your ship for XC.

Besides basic flyability of the glider the next thing to get set up properly is a perfectly working trailer and then a pee tube.

A thermos can smack you in the head in severe turbulence or roll around and potentially block the rudders. You just don’t have space for a cup holder.