These were / are actual rifle scopes. They have a set of lenses in the tube. I doubt very much you could make one yourself, unless have some experience constructing multi-element lenses. You will need an optical bench to space the lens groups properly.

You can purchase reproductions rather easily. Pedersoli makes several. http://www.davide-pedersoli.com/accessoriDettaglio.aspx?CategoriaId=1317&lang=en

Dixie Gun works may have some. http://www.dixiegun.com/

These type of sights were used during the American Civil War (1861-1865). They mostly were used by Union snipers. This was because of the expense involved at the time for such a sight.

Pete