Just got my game on Monday, so I'm not sure what discussion there has been about improving the projection quality (EDIT: Have now gone back to see it has been discussed plenty , but perhaps this will help someone anyway), but after experimenting with over 10 different materials, including various vinyls and actual theater screen materials, this Cricut silver proved to be the best of those (better options may exist): https://www.amazon.com/Cricut-Joy-Smart-Vinyl-Permanent/dp/B084LMT7LR/ref=sr_1_8 (Got mine at Michael's)
It enhances the black levels as well as the brightness/white levels and color vibrance.
Most darker materials (even other silver materials) would enhance the black levels, but generally muddy everything else (unless angled more toward the player, then that scenario produces a stellar image on Siser's Chalkboard black, but doesn't work on the flat factory surface).
Unfortunately, the ramps are too steep to get a better image than the white with my testing thus far, and while it makes the standup targets looks nicer, the insert reflects more significantly off it than the factory white (though the factory standup reflects some as well).
I have one more material to test before doing my swap, but don't have high hopes for it.
Test pics (the deeper black and brighter whites in the 'RUN!" text is a fairly clear indicator, as well as the more saturated colors):
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No longer recommending the Cricut silver (too much shimmer when fully installed). Have more materials coming to test soon.