HandiRifle: As said, I really think that you should also pose your question over of the American Single Shot Assoc &/or the Wyoming Schuetzen Union boards. There are some every knowledgeable & experienced long time shooters over there. Maybe even look @ the Nat’l Muzzle Loading Assoc board?
I looked in G.O. Kelver’s “Reloading Tools, Sights and Telescopes” (1982). For Unertl, the info on the ‘tube sight was sketchy @ best: no optical system, an "adjustable peep” (no explanation as to what this means) to regulate the am’t of light on sight & target, used in conjunction w/ front sight.
Kelver claims that some shooters also replaced the front mount w/ a rear mount to keep the sight parallel, but this was only necessary for shooting outdoors @ distances greater than 100 yds. This was the only information that I could find on tube sights in that book.
As I suggest earlier, give Gil Parsons a call. He’s been in the ‘scope & mount fabrication & repair business for many years.
Given your visual problems, would a tube sight work for you. My eyesight isn’t the best & iron sights are extremely difficult for me to use --even @ 100 yds. I “loose” the front t sight & the target is very difficult to see.
If you took a piece of tubing, cut out a disc of cardboard w/ a hole centered, put this on the end of the tube, then mounted this via spacers & tape to your rifle, & “sighted it”, would you get an idea as to whether you will be able see the target & /or the front sight w/ a tube sight??
Are you planning on mounting this on a NEF Handi Rifle? Over on the Shiloh board you made some posts ‘bout rebarreling a Handi Rifle to .38-55??