I figured I would pass this on for anyone else with a Data East Star Trek 25th Anniversary pin looking to brighten up their transporter display.
I'd been experimenting for some time looking for just the right material to replace my crumbling mylar diffuser that sits between the fluorescent light and the shifting glass of the transporter. I finally found the perfect 12"x18" sheet of white mylar at my local Jo Ann Fabrics store for a mere $3! This particular piece is made by Dritz, part #3117.
As I mentioned, it is white mylar and is the the exact same thickness (.01" as the original sheet. My old one was brittle and cracking from age, but hadn't realized how yellowed it had become until it was laying on the table next to the new one. I regret not taking some comparison pictures at the time.
Afterward, I can only say Wow! It made a huge difference. It went from a softer warm white scene to a significantly brighter cool white. If you have one of these machines, you might want to consider doing this inexpensive restoration. It was much bang for the buck in my case.