Bicycle grips. I found some on ebay for $5 that had a diamond texture, and firm rubber. They are working and lasting well. Get ones that have the end plugs as you will need these for the PTT switch mounting.
I used a dremel with a fiber reinforced cut off wheel to cut close to the scribe lines, and then finished up to the scribe lines with a sanding block made by glueing with contact cement some 80 grit sandpaper to a 2x4. This worked very quickly and accurately.
I spent the money for RC Allen grips with a good PTT switch at the top. (No other switches).
To trim the fiberglass pieces, you will be REALLY HAPPY to use the following rather than the Van's suggestions in the plans :
1. A Dremel with a small thin cutting disk - about an inch in diameter to maybe 1.5 inch.
2. A 4 inch bench-mount belt sander with a coarse to medium belt and the curved end of the belt exposed on the end (see link). This is the most important tool! You can cut almost to the scribe lines with the disk then use this and it is very precise - believe it or not. Saves many hours of hand sanding. The long flat section is important.
3. A one-inch belt sander (see second link) is very useful for a variety of things in building the RV-12.
I did the Ray Allen grips with improved PTT as well, I believe it is a wise choice. For a few bucks you can get an extra grip and it fits on the flap handle as well.