I have a Brandon Master Birder lens cell, with rubber housing. This 80mm objective is advertised as f/5.6, however, Vernonscope has confirmed that f/5.6 is wrong and with a measured focal length less than 400mm, f/4.8 is more like it. It is also advertised as a triplet, and to be more accurate, it is an air spaced doublet with a flattening element about 12mm behind the doublet. The reviews say the image breaks down with power, and that's probably the tradeoff for the flatter field. There is less field curvature at a very fast f/4.8, which is nice for terrestrial viewing and maybe the ultimate rich field telescope.
I have adapted an Astro-Physics 2.7" focuser extension to mate with the lens cell, therefore a tube can be fashioned from these extensions or there is enough stock to fashion many tube solutions. The OD is 78mm or 3 1/16".
You'll need your own focuser as well.
$160 shipped CONUS. Money order please.
Thanks for looking