For awhile I tried clip on sunglasses over my smallish lens hidden
bi-focals. The problem was that in shade they were too dark, and
because they are polarized I had difficulty seeing my tippet to tie
knots. Because they were clipped on (not the flip up type) I couldn't
simply remove them every time I fished in the shade, and had to remove
them at dusk and didn't know what to do with them when I removed them.

So I dug out a very dark, kinda expensive at the time, large lens
flip-up sunglass that fit my very old aviator style glasses. I also
dug out a cheap version of the flip-up style in the same size lens.

Starting with the cheap pair, I mounted the flip-up and traced the
outline of my small lens eyeglasses. I then took a small scissors and
cut out the smaller shape. The edges got a bit rough, but a little
filing and then a swipe with acetone along the edges and I now had a
really small set of flip-ups. The expensive pair was much thicker and
ended up rougher after the same treatment, but with some fine filing
and the acetone swipe they too looked decent, well good enough for
fishing.

At their new size they are very light and flip up when not needed.

Jeff Shafer