I just installed a 50" fence on my saw and tuned it up. I don't even want to mention the brand its paid for. I bought years ago trying to make a piece of crap saw work. I gave up before I even opened the box. It's been long enough that I decided to try again. I bought a good old saw and did my first project with little set up. It came out pretty good joints could have been tighter.So I bought a new miter gauge the old one was loose in the slot and the head was loose.I also blew the dust off the fence 3 years old still in the box. tuned it up within a ch.Now the question if yur still awake what is true enough to use as an aux fence.
That all depends on what you want to do. If you just want something there to take a dado blade abuse for a rabit then almost anything will work. I would go with a piece of MDF and attach it to the fence and go with it. I use an old piece of hardwood that was about the right size. As long as it is uniform in thickness it should not matter much.
Lou is right on Dave.
Most of us use some sort of a "sacrifical board" on a fence or miter square.
Easy to replace if you want to.
I'm a MDF fan myself. You slice off an 1/8 " stick of it and you can make a circle out of it. It's like rubber when thin.
I use it for all sorts of jigs and fences I figgure I'll wreck eventually.