You could use a sheet of circularly polarised filter material- you lose 1/2 the emitted light, but about 98% of reflected sunlight - could well do the job, and it's a neutral (grey) colour. Maplin used to do it, Farnell definitely stock it.
I know where you're coming from on the open top glare issue, although the graphichs smear massively in mine, as my retinas are shaking at roughly the refresh rate of the display :)
The green filter is the best stock option for sunlight visibility, by the way.

Steve