A follow up on this one. I did a lot of internet searches and phone calls and could not find an awning cap or even a used awning of that size. So I was getting ready to make a fiberglass cap and a friend said why don't you see if you can find a plastic bottle about that size, cut off the bottom and see if you can make it fit. I tried several different bottles and none came close but noticed a small 3 cup Glad storage container looked about the same size. I trimmed the container, got out a heat gun and applied a little heat. After messing up the first try I went to a local store and bought four more containers. After a few more attempts I was able to shape a container to cover the end opening. Painted it a matching color and it looks pretty good. I added two screws to hold it in place. I learned to be a bit more patient with the heat gun, once that plastic starts to shrink it can ruin the new cap almost instantly.
Bob, Sandi & Marmaduke the Big Pug
SW OREGON 2004 Journey 39K, 330 Cat
If towing: a 2006 Mini Cooper or 1995 Wrangler