I made a router table insert from aluminum plate salvaged from a parking sign. I don't know what alloy they use, but it's hard and stiff. It was pretty warped when I got it, so I had to peen it flat. I've been using it for a couple of years and it's served me well, but I want to replace it with a commerical plate with inserts. Guess I'll be making a new top, too.
Because it's only about 1/8" thick, I made as small a cutout in the top as I could.
Here's my new (April '02) Rockler insert & top.
If you look closely in the picture below, you just might make out the old top leaning against the waste pipe.