I think the hole in the ceiling of your Airstream is a missing Astrodome (sometimes spelled Astradome) skylight. Both Vintage Trailer Supply (www.vintagetrailersupply.com
$159.95) and Inland RV (www.inlandrv.com
$137.95) carry the size I believe you need. I ordered a replacement from Vintage Trailer Supply and am OH SO HAPPY with it. (My other was crumbling from age.) Vintage Trailer Supply has some installation tips for you online, as well.
If you are interested in viewing some other Airstreams, your make and year, you can search www.vintageairstream.com
for photos in their archives.
Hope this helps!