Couple other ways:
Take the damper flap and rod out of the damper body and take it to a hardware store. Find a small coilspring that will slide over the damper rod. Find a small E-clip (C clip, retaining clip, someone will know what it is if you don't) that fits the control rod.
Reassemble the damper flap and rod into the damper body, then slide the coil spring on the outside of the control rod. Back it with an E clip so the spring has tension. This works really well but you will burn through the spring within 3-4 fires. They cost cents per piece, so a ziplock full of them in the stove bag doesn't cost much and weighs very little.
Method two: Bend the "wings" on the damper flap so they make very light contact inside the damper body. This has differing levels of success on different flaps.