If you are only worried about cosmetics, then a piece of rubber or vinyl or balsa wood will do the job. But if there is an accident, and your head hits the edge with any force, there is a good chance the aluminum edge of the glareshield will cut its way through the edging, and your head will be sliced open.
If you want to protect the head, you need to put something hard and rounded over the edge of the glareshield. I have seen postings on various e-mail lists from people who cut a slit in the edge of a piece of aluminum fuel line, and put that over the edge of the glareshield. Then you put something over the fuel line to improve the cosmetics.
The following is from my personal archive of useful RV-List messages, and was originally posted by RV-6 builder Mark Nielsen:
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For my glareshield, I mounted a piece of 3/8" soft aluminum tubing (same as
fuel line) along the aft edge of the glareshield. I cut a lengthwise slot
in the tubing, pushed it on the aft edge of the glareshield, and held it in
place with fillets of epoxy along the top and bottom. I was pleased with
the results. It gives the glareshield a nice "beaded" edge that looks right
in place next to the tubular frame that supports it (tip-up canopy). The
tube also stiffens the glareshield considerably and eliminates the "sharp"
aft edge.
To cut the slot in the tubing, I used a 3" cutting disk mounted in a drill
press. I drilled a 3/8" hole near the edge of a 2x4 and cut a slot between
the center of the hole and the edge. This was clamped vertically on the
drill press
so the cutting disc protrudes through the slot into the center of the hole.
The tube was pushed through the hole to cut the slot. (I drew a guide line
along the length of the tube so I could cut the slot straight.)
I used Power Poxy (Walmart) for the fillets. It has the consistency of very
soft modeling clay, is easy to use, sets up in about 30 minutes, and sands
easily. The application tool was my finger moistened with rubbing alcohol.