Having her be able to drop her head and graze is going to be a losing battle trying to keep it covered and clean. You can bandage it by first shaving the area around the wound at least 2X the size (about 8x4) Wash it, dry it, apply what ever type of salve you want on it. Apply a non stick gauze over the wound than take 3 or 4 inch elastikon and run it around the edges of the gauze making a extra large band-aid. (2 inch tends to roll and bunchup on horses) Rub and warm up the Elastikon so it really sticks to the skin. It will stick to the skin better than hair. Get a fresh roll of the stuff. If it sits around it loses it's ability to stick. If you can get her to drop her head while you apply the bandage than you will stretch the skin out and won't get the pulling action on it when she lifts and drops her head and should stay on longer.
There is a spray on bandage out there or as others have said just let it go without. Put enough goo on it so dirt doesn't work into the wound but then you have to wash it and reapply daily.