Flour makes a GREAT powder substitute. But if you're looking for a really good substitute, that's soothing to the skin, fairly cheap to use, and is made from common ingredients try:
3 1/2 cups of flour
1 teaspoon of sugar
1 envelope of instant dry yeast
2 teaspoons of salt (kosher is best)
2 tablespoons of olive oil
Mix the dry ingredients together and sprinkle some on the diaper before applying. Prepare your diaper area by applying the oil. Fasten the diaper as normal (be careful that you don't get oil on the tapes!) and then proceed to pour the rest of the dry powder in the front and back of the diaper (add a couple more teaspoons of oil in the front and back too!). This mixture is best applied when you're fit to burst with a full bladder!
Once you've applied the diaper and all those ingredients, you'll feel so good and relaxed that you'll want to instantly empty your bladder. Do it! Man what a rush! Feel free to enjoy the full body sensation and the new feeling with your powder. It's amazing if you kneed the area fairly well too!
On a more serious note: Baby powder is quite common for some guys to use. I used it like crazy for my socks, boots, and "underwear" when it's hot out. It was pretty common for guys to do this in hot environments when I was in the military still. If you're concerned about the scent, use unscented powder.