Candy cane closure for trash compactor bag:
I couldn't find a picture. Here's how to do it:

Gather plastic bag together at the top. Twist tightly.
Fold the twist over for several inches--that's the "candy cane."
Tie cord or fasten rubber band tigtly around the doubled twist.

If done right, it will be waterproof. When practicing before the trip, test by immersing in the bathtub.

Take extra fasteners. Rubber bands are more effective, but they tend to break and/or pop off into the brush when fastening and unfastening. The cloth covered narrow elastic bands used for fastening ponytails hold up better and are a bit easier to find.