The JP Jpoint sight is very compact, super light, and has a number of pistol mounts available (
http://www.jprifles.com/1.6.1a.php).
If you are doing a custom artillery-type barrel, I would set a short (2") picatinny rail where the rear sight customarily sits and place the sight on that. If you want lower profile and eliminate the rail, JP has a gunsmith mount that allows you to attach the sight directly to the barrel (
http://www.eabco.com/store/index.php...t_version=true).
The Jpoint mounts will work with the Tasco Optima as well. If you are interested, I have a spare Optima sight.
--Rootshot