The number you want is KM610.
B&B Aircraft sells them for about $5. You can call them at 913-884-5930, or here's are a couple of threads that talks about them:
I ordered two from him and you'll want the size that is for the greatest thickness, if I remember correctly. If you tell him it's for an RV cowl he'll know what size you need, or someone else can post it. (My receipts are not yet organized enough to tell you!)