The best results I've gotten are with balsa. I bought a simple 36-inch length of 2X3 balsa on eBay and cut it in three 12-inch lengths with a miter saw. I treated one with .25-micron diamond spray from Hand American and the other with .3-.5 micron CrO from the same source. All other things being equal -- in other words, honing -- with the help of those blocks I now get edges that I pass through my arm hair about a half-centimeter above the skin, and the hair falls down like mown hay.

In my experience, having the diamond lapping surface rigid is one key; the second is having the material slightly springy. My first lapping block was oak, and I didn't have such a good experience with that. Balsa is forgiving. I suspect denim would do pretty well if attached to a hard, perfectly flat surface.