I had some acrylic sheets from old Melissa and Doug toy boxes, and needed some arm protectors for archery for myself and the kids (I later used more of the acrylic for a bow sight). A bit of work with hair dryer, box cutters and rotary tool, and I made some that work fairly well, though they are somewhat difficult to put on over clothes (Velcro might work better).
The arm protectors have two functions: one is to protect the arm from string slaps, and the other is to keep clothes out of the way of the string. The smooth plastic also may mean that if the string goes on the protector, it won't distort the shot extremely (though it will distort it significantly).
These are usually made of leather, but I don't have any scrap leather.
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Step 1: Cutting Acrylic
Cut acrylic sheet to the length you want, and wide enough to wrap as far around the arm as you want it to.
You can cut acrylic sheet by scoring deeply (a couple of passes may be needed) with box cutters, and then breaking (while wearing safety goggles, of course). Then you can smooth the corners and the edges with sandpaper, a file or (in my case) a sanding drum on a rotary tool or drill. As always, when sanding plastic (and other things) wear breathing protection.
Step 2: Rolling Acrylic
In a well-ventilated area, blow a hair dryer from close proximity at the sheet, on both sides, until it becomes sufficiently pliable. I bent it against carpet until it attained the right shape to fit the arm--wider at the upper end.
If you bend too sharply it will make an ugly crease. But in some places to do the right bends, I did have to use plyers on it.
Step 3: Make Holes for Elastic
I cut elongated holes for the elastic along the edge, about four per side, by using a burr on a rotary tool.
Step 4: Insert Elastic
Now thread elastic through in the way you like. You can tie it off, but you might not actually need to if the holes are snug enough.
Step 5: Final Trimming
Now put it on, bend the arm, and see if comfort requires further trimming. E.g., corners may need to be rounded more. I also found that my protector became more comfortable if I trimmed it at an angle near the elbow.
This final trimming is actually a little harder than the initial cutting. Initial cutting is done by scoring and breaking (and then sanding). But when you are only cutting off a thin strip, it's harder to break off. I used pliers to tear off the strips.
As always, when sanding plastic (and other things) wear breathing protection.