So this is a fast, cheap, super easy solution... with no flapping loose corners. And it’s unlikely to be stolen! The original lasted almost two years and was discarded only because I was too lazy to clean it and I needed to make a new one for this instructable.
Step 1: Materials and time needed
1 garbage bag with drawstring
3 spring clothes pins
Less than 5 minutes
Step 2: Bag Detail
Note that the sides of the garbage bag are heat fused. The drawstring is anchored at both sides by this heat seal. You will need to cut BESIDE the heat seal, not through it.
Step 3: Cut one layer of the bag
Leaving the heat seal on both sides intact, Cut only the top layer of the bag . Cut both sides this way from the top to the bottom. DON’T CUT ACROSS THE BOTTOM!
Step 5: The one slightly tricky step
On the cut end of the sheet, carefully push the bag back from the cut ends of the drawstring on both sides. If you pull hard the drawstring will just slide all the way out... (If your hands are big, it might help to use tweezers to get the drawstring out far enough to grasp.)
Step 7: Pull the still heat fused drawstring at the opposite end
On the end of the sheet with the heat fusing, pull the drawstring at the center. You should now have something vaguely resembling a floppy plastic canoe.
Step 8: Cover your handlebars
Put one end over your handlebars. Adjusts the drawstring to fit. Use two clothespins to secure it to the brake cables.