Boring a deep hole with a Forstner bit requires a bit extension. I made this Fostner bit extension at the TechShop San Jose, http://www.techshop.ws, using the Jet Metal Lathe and Jet Vertical Mill.It is turned from a piece of 3/4 inch diameter X 8 inch long cold rolled steel, the type available at HD, Lowes or your local hardware store.
Start by mounting the stock in the three jaw chuck on the Jet metal lathe and face off the end. Proceed to turn the diameter to approximately .45 inches dia. X 1.25 inches long. The jacobs chuck I will be using this extension in has a nominal .5 inch maximum opening and I want to keep the end of the shaft as large as possible, so I turned the shaft to .45 inches diameter.