Kevin
If that RB has the Pedersoli sight on it, you can get the adjustable "Hadley" disk for it from Cabela's. It has (I think, I'm too lazy to go downstairs and count thiem) 5 or 6 different sized holes to look through. The combination of my eyes and glare from the top of my Sharps rendered me blind looking through my sight before I got this. It's a lot cheaper than a Parts Unknown, MVA or Baldwin, too.
Most sight makers have a Hadley eyecup available, so if it's not a Pedersoli sight, you can probably find one to fit.