I know of several methods for dealing with presbyopia that do not involve modiifcations to the rifle and a few more that involve minimal modification. I'm sometimes surprised they are not better known.

In my own case I have a pair of glasses made up for shooting doubles. They allow me a sharp view of both the front sight and the target. The prescription scared the counter girls at the eyeglass store but we're not all that worried about what they think.

I have another pair made up for use with a handgun.

if there is interest here I'l lgo into detail.