Pete,
You may be better off investing in a tension gauge. I have a Loos gauge that I bought many years ago and I use it once in a while when I want to check the tension of my rig.
The gauge has a series of numbers that correspond to various tension choices based on the thickness of your stays. You may have a choice for about four different numbers and what you do is set your upper stays to one of those numbers. Your lower stays should have a little less tension than your uppers. So you pick a number and tension them with that number.
Myself, I like this method because it takes all the guess work out of tensioning your rig, plus the fact that the equal tension of the stays all the way around is going to insure that your mast will be straight when you get through.
I may be wrong but the gauge is easier than using your main halyard with reference points to measure the straightness of the mast, and laying on your back on the sliding hatch trying to sight up the mast for straightness. Then they tell you, "Well you tighten up your turnbuckles until you can hear a "plongggg!" when you pluck it with your finger." That's like trying to read those old instructions on how to tune an Ukelele by singing "My dog has fleas."
Like the man said, Rudy at D&R Marine would have these Teak hand rails and slider moldings.
A friend of mine replaced his Teak rails and slider moldings with StarBoard on his Seaward 22. He also built companionway pen boards out of it. I think that Trex would be a good choice also. I've used both Trex and PVC trim board on my boat for certain things and it worked out fine for me.