My "router table" consists of a piece of 3/4" MDF on a couple of sawhorses. I haven't had trouble with bowing, but the experts would probably recommend doubling the thickness and, if you really want to get fancy, surfacing the top with laminate.

My fence consists of three lengths of 3/4" MDF glued together.

A propos of nothing, my Woodworking 101 instructor even had a portable router table, consisting of a four-sided (top, bottom, two sides) unfinished MDF box just large enough to accomodate a router and a length of MDF for the fence. He clamped the bottom of the box to the workbench or Workmate, clamped the fence to the top of the box and was ready to go.

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