Lots of times on woodworking shows they make jigs or adapters for their routers using 3/8" or larger plastic.  The Woodsmith show does that a lot.  I am building a large door and it would have been nice to make an adapter for my router to make the mortises but I can't find a source of plastic like they use.  I don't need a 4'x8' sheet or special acrylic and I don't need a lot of it. 

I suspect I'd not be making a base much bigger than 8x12 ever.