I've had great luck with titebond II glue when gluing up 2 sheets of MDF. I've used this approach on one of my earlier router tables, as well as my current work bench.
The technique I use is to clamp around the edges, and drive drywall screws in the center of the field (from the underside) to act as "clamps". If you're worried about having screws or screw holes in the bottom of your router table, just drive them in the area you are going to be cutting out for your insert plate.
When drilling holes like this in MDF, it is important to not only drill a pilot hole, but just slightly oversize the hole in the upper piece of stock. Otherwise, as the screw cuts into the lower piece, it could push material up and create the same void you were having using gorilla glue.
As for contact cement, I guess it would work, but just make sure that you leave one of your pieces OVERSIZED. In case you don't get your line up just perfect, you can chuck up a flush trim bit and no one will be the wiser.
Hope this helps,