Brian, what size monitor screen. I also did this with mine and you can get screen protectors with anti-glare that do not darken picture. I did both my ST16 screens and my ST10 screen along with the extra monitor screen. My aux screen is 5".
These come 2 to a pack. They are for ST24 but same as ST16. They can be carefully cut to size or look through the listing for the size you need. I have not found any better that actually do the job we need in bright sunlight.
I use exclusively the Brotect screen protectors, protectionfilms24.com