This is not a true Instructable, but rather an idea that you can adapt to your needs.
I used to have a router table, but had to get rid of it due to space limitations. A few week ago, I really needed a router table and I don't know why I hadn't thought of doing this sooner, but I have fitted my router into my Radial Arm Saw table. I took photographs of what I did just to illustrate what can be done.
Step 1: The Gist of What I Did
First, I cut and drilled a template and support to suit my particular router out of 3/8" Plexiglass, then I routed a hole and template recess in my table to match the router and the Plexiglass template. Be sure that the template will fit perfectly flush with the table surface otherwise when using the router or the saw, your work-piece will catch on any uneven surface.
I then fit the Plexiglass template into the table. (be sure to drill pilot holes in the wooden table for the mounting screws).
The router was then mounted under the table and again check that all of the mounting screws are flush with the table top.
I also mounted a switched and circuit breaker power bar on the side of the saw frame so that any tools could be plugged in locally. This was not only useful, but also necessary for switching my router on/off. My router has a spring-loaded on/off switch and I have put a tie-wrap around it to keep it in the on position when using it in the table.
I use a small 'rare-earth' magnet to hold the router wrench onto the steel saw frame. When a guide is required, I clamp a wide piece of wood to the saw table. Using a narrow guide may distort when shaping a work-piece.