Here's three wine racks made from the same tree (a Douglas Fir which was about to fall over on our barn, so had to remove it or risk losing the barn). Each is 6 inches thick. Drilling the holes was the hardest part of these projects. I tried a 3 5/8" self feed bit, but clocked myself pretty good in the shin (twice - I'm kind of a slow learner) because of the amount of torque being applied, so went back to a three step process of using a hole saw, a router, and then the hole saw again to get through. I had to go at it from both sides of the slab which is a lot of work, but the results have been well received by everyone who has seen them so far. Sanding in the holes was done with a simple flapsander wheel I bought at Ace Hardware for $5.99, and it worked great.