It's a matter of personal preference and how you use it. 1/2" will work fine, but if you try and load it on a flat surface you'll likely be shaving the top of the balls when cutting off the patch. I like mine about 3/16" thicker than the balls. That way, I can seat the ball with the knob of my short starter so that it is just below flush to slice the patch and then ease the ball in just a tad farther. Then, when I'm hanging it off my pouch or from around my neck, neither the patch in front of the ball or behind it sticks out proud with grease to rub on my clothes.
Some folks get super fancy and use a big 'ol log with countersinks for the muzzle. I just *** my thumb in on top of the ball when I want to load it so that the ball sticks down enough that I can feel when it rests over the muzzle.
I like five shots per block, and usually carry one in the bag and one around my neck. I do almost all of my shooting 'from the block' and re-load the blocks rather than patch at the muzzle.
Another tip: If you tie a thong to the block, lead the thong out the side(s) perpendicular to the bullet holes. Otherwise, you're apt to slice the thong when cutting off the patch (I know this).
Walnut and maple work best, but any wood will do. One of my favorites I made out of purpleheart, but it seems to splinter around the holes more that the others. Every pine block I ever used split, but then I drill my holes only about a half-a-caliber apart.
