NavList:

Perhaps there is a solution to your filter  problem.   Kodak makes a neutral density filter (Wratten #96) that attenuates light fairly uniformly across the visible spectrum.  It is supplied in gelatin sheets that can be cut using a pair of scissors, and could be cut to fit your index filter.

The filters come in 0.1 logarithmic steps, 0.1 to 0.9 log units, and 1, 2, 3, and 4 log units.  You are describing a 3 log unit filter, that is one that transmits 1/1000 of the incident light.  The filters are additive- adding 0.3 log unit filter to your filter will make the sun's image half as bright.

I'd suggest mounting between two thin flat pieces of glass.   Not at all sure what kind of distortion this might produce if the sandwich is not perfectly flat.  I've used Wratten filters for decades measuring visual detection thresholds.

Caution about exposure to direct sunlight is warranted.  I have seen retinal lesions caused by solar retinopathy.

Hope this suggestion from a vision scientist and amateur navigator leads to a solution.  Best wishes.

Ron Hansen