First thing would be to hunt down the proper sized tube that would match the fan's size, and to have the fan's hole already cutout of the metal case front.
Then carefully mark and cut the hole in the bezel to the outside diameter of the tube, and insert it. Measure carefully (more than once to make sure) because you want the hole to exactly match the fan's position.
Place the bezel back on the case, insert the tube up to where you want it, and carefully mark it around the tube-remove-cut. Reinstall the tube and glue it into place from the inside of the bezel.
Then sand/dremel to a nice finish and paint.
Diggrr has the right idea. If your looking for the "right size tube" Check out all your old cottage cheese, cream cheese and small butter containers. You know...all those little plastic containers. They make great ducts. Take paint well if you sand them a little bit to rough them up. I used a small country crock butter tub as a "tube" to cut apart with my dremel and then used epoxy to stick it to the case front. I painted it silver and it turned out that one of my 80mm fans fit perfectly in the bottom of it. I then just stuck a 80mm fan grill over the front and it turned out looking half way decent.
Once you find the right size of whatever you are going to use for the hole and insert it you can use bondo to cover up the edges and sand it to look like its built into the case.You can find bondo or also called body filler at just about any automotive store,I think kmart and wal-mart even carry it.
I just noticed at work while packing up a system, and had just run out of tape. I was just taking the empy tube off of the tape gun and threw it in the direction of my trash container. Luckily I had particularly bad aim that day and it hit a spare 80mm fan I had on my test bench and knocked it to the floor. I walked over to pick it up and fetch the stray tube at the same time. Man can those things roll! So with empty tape tube in one hand and fan in the other I start walking back to "place" the tape tube in the trash this time. That's when I noticed that the tube was the perfect size for a duct. So instead of going in the trash, I got an empty box and started collecting them. No clue what I'm going to do with them all, seeing as I only need 2 of them. But who know's might come in handy sometime.