Bean Man:
Build you a firebox, like a woodstove, that sits outside of the cooker. Connect the firebox to the cooker by a stovepipe, with a damper between the firebox and the cooker. Pipe the stovepipe into the bottom of of the side of your cooker opposite the stack so that the smoke passes through the cooker.
Have a draft on the door of the firebox so you can control the rate of burn. Put a metal stem thermometer somewhere either set into a well you weld onto or more simply a hole drilled into the lid the stem of the thermometer fits tightly, so you can monitor the temp. inside the cooker without having to open the lid. Control the temp inside the cooker with the draft and damper of the stove. My setup has another damper at the top of the "chimney".
You should be able to extend the cooking time at least a couple of hours and you'll surely get plenty of smoke too. I try to keep the temp at 200 + or - 20 degrees.
A drain in the bottom of the cooker for the excess fat to come out of, into a collection pan, is nice to have too.