Countersink with a drill bit and the screw will be flush if you do it right, just google how to countersink metal and I'm sure you will find something, super easy.
I thinkk trex would get chewed up pretty bad.
See if you can find 3/16" HDPE from a local plastics supplier. Might be cheaper than steel, definitely easier to work with and will slide better. Not sure about the corners of the rail though.
I'm buying a ton of HDPE, so I might have some left over for a flat rail. Not sure how I feel, It would kind of just be like sliding a small box, right? Also, I've seen a lot of people use PVC Pipes as the topping, how would 3 PVC pipes side by side work on a rail?