Hi Barry, long time no see :-)
I think the choice is between un-bendable, narrow strips thick enough to recess a countersunk screw or rivet, and that means 1/4"; or thinner, bendable strips that will wrap around a spar, to be fastened above and below the contact point, and that means about 0.125" for a 2.5" spar. I wouldn't want to go thinner than this, because of the risk of tearing away from the fastener. Thus, I wouldn't do a wrap around strip on a small diameter batten.
The first option has been used successfully. For a small diameter batten, I would cut a strip from 1/4" sheet of a width equal to the diameter, so that it is not inclined to rotate when pushed against the mast.
On the new sails that we have just made for Footprints and Tystie, we have built the fenders onto the batten pockets, in the form of strips of closed cell foam 3/4" x 5/16", underneath 2" car seat belt webbing, which is hard and shiny, and resists wear.