This is the substrate for my new router table. It consists of a layer of 3/4 Birch Plywood on bottom and 3/4 Medium Density Fibreboard (MDF) on top. It is stiff and flat, two things one really needs in a router table. The whole thing will be topped off with the big white sheet you see in the upper portion of this photo, and I'll explain that in a minute.
You can see the outline of my router plate traced onto the surface with a hole cut into the center to accept the router itself. I cut the hole with my jig saw. I own the worst jig saw in the long and storied history of power tools, by the way. I think I shall recycle it as a paper weight at the first available opportunity.
The actual surface of my router table will be made of 1/8" Melamine hardboard, which you can see in the first photo. I have laminated a sheet of 1/8" hardboard and a sheet of 1/8" melamine hardboard together because the glue line binding the two creates a stiffer product than 1/4" melamine hardboard alone. However, I still have to add a very thin layer of... something I have yet to identify, in order to raise the melamine to dead-level with the surface of the router plate. Or I can cut down the surface of the MDF under the plate so the plate will drop down to be level with the Melamine. I'll do with this what I do with most of my projects: I'll figure it out as I go along.
This is the jig I built to guide my router when I cut the hole in the melamine for the router plate. The glue is still drying in this photo, which accounts for all the clamps. The melamine will be mounted on top with the hole already rough-cut into it, then the router, fitted with a straight bit with a bearing on the bottom will be guided by the jig from underneath.