That's not a bad idea, maybe something out of styrene.....following!

I think it would depend on the game that would dictate if you would have enough clearance to raise and lower the playfield without digging into them with any protrusions from the side of the playfield. I have game blades on my Dirty Harry and it just clears and I even have to hold a sheet of poster board between one side when raising the playfield to keep from gouging the blade. Adding another layer under it would definitely be too thick. Also had mirror blades on a BSD I used to own and also was a very tight squeeze. Had lots of tiny scratches from raising playfield. Game blades on top of something of that thickness probably would have gotten destroyed on that game anyway.

Does it have to be a reflective material? I picked up some vinyl wrap for my STTNG and used it as a backboard material and plan to do the same with the sides of the cabinet.

ebay.com link: Rwraps Galaxy Vinyl Wrap Sheet Film Roll for Custom Body Kits More

You can buy large sheet styrene like that is used to make advertising signs in the same thickness or thinner that mirror blades, and then easily cut them since they are styrene.