If you think about it, Richard, the fence is just a surface to guide the wood past the bit. But, there are a couple of criteria that must be there: it needs to be very straight, and it needs to have a face that is precisely square to the table surface.
My first router table was simply an old cutting board into which I inlet and attached a router. The fence was simply a jointed 2x4. It worked fine for simple edge-forming operations. I later "graduated" to an inexpensive (relatively speaking) Rockler table top and fence combo. The Rockler had a split fence - that is, the face was split, and mounted to a piece of anodized aluminum angle. As such, the outfeed side could be shimmed with plastic sheets for such things as using the router table as a small jointer. Not bad, but the aluminum angle wasn't very wide (or, thick), so it didn't provide a very solid base for taller face boards.
When I built my current table, I decided to make my own fence using 3"x3"x3/8" thick aluminum angle from a metals supplier. The faces of the angle had been machined to a precise 90° angle, so that gave a high level of precision for the fence, and the height and thickness provided a solid base for mounting MDF face pieces. I made two sets of face pieces - one set is 6" high, and the other is 12" high.
There are also commercially-made fences available, including the Incra "precision" models. Decide what you want the fence to do, and then use that as the basis for deciding which way to go.