Hey geekygrappler,
Almost any wood with straght grain makes a good batten. Some hard woods might be too rigid. I used 1/4" x 1/4". I take it from your post you don't have a table saw? A batten really must have uniform thickness and width or it won't bend fair. You could actually use some of your 6mm (1/4") plywood, but again, it would need to be ripped uniformly.
The easiest way to bend the batten is to put the nails on the line and bend the batten around the outside of the curve. This will give you a fair line on the inside of the curve, and it will be the exact line. Just trace it lightly so as not to move the batten. Cut just outside of the line, and tweek or finesse to the line with a block plane or sanding block.