When mounting a lens flange in preparation for use you need to make sure there are no light leaks. Light can seep in through the lens flange and your images will be fogged. Here is a cheap and quick photo illustration of how I make sure my lenses fit nice and snug against the lens board.
At your local hardware you can pick up sheets of rubber this one was $5.00. At Dale McFaddens they sell rubbers already cut to size, and you can take your lens and usually find the perfect fit. I used the rubber because the hardware store is down the street.
You will need screws and nuts to hold the flange tightly to the board. Take your lens board and flange to your hardware store and buy the screw that will fit in the holes. You will also need to pre-dremel holes into the lens mount so you can easily fit the screws in place.
Cut the rubber to size, dremel holes into the wood, and screw in the screws and apply the nuts and you are ready to go!
Since lenses are worth so much now a days it is best to make sure everything is on tight and securely. The rubber was only $5.00 and I can use it for two other lenses I have and the screws and nuts were only a $1.00. Simple solution that will save you a headache in the future.