This style camper hatch may put you in trouble for rot:

We special ordered the drip cap thru a dealer that uses this source for their RV products
http://www.go-rv.com/coast/do/catalog/page?dealerId=2642&pageNum=254

Go here to select your state or province, to see the dealers in your area that use this same resource. http://www.go-rv.com/coast/do/chooseState

Available in lengths from 9 to 73 inches. The length we needed was less than $5.

That catalogue can be somewhat addicting, careful!