Quick & Cheap Cup Holder
For $2.55 at Ace Hardware, plus two plastic cups available at Walgreens six for a dollar, you can make two cup holders. They are rustproof and do not mar your handlebars because only plastic touches the paint.
Buy an Ace Hose Mender 5/8 - 3/4. This is part number # 74891, costs $1.99, and is two yellow clamps plus a black plastic pipe you don't need.
Buy four stainless screws #8 one inch long, for 14 cents each. The screws provided with the hose mender are not long enough to go through a cup and screw into the clamp.
Unscrew the clamp and set its screws aside; you won't use them.
Use a pencil or an awl, through the clamp, to mark where holes go on the bottom of the cup. Drill those two 1/8" holes and a couple more for drainage.
Put screws through the bottom of the cup and the upper part of the clamp.
Position the cup on the handlebars where you want it. I suggest just right of the left mirror.
Put the lower part of the clamp in place, tighten the screws, and you are done. Make the screws just tight enough to hold the cup holder firmly in place. If you strip the lower clamp, then use the other one and be more careful.
If you didn't break or lose anything while doing this, you have one for your other bike ... or for a friend's bike.
Good luck, and bottoms up!