Converted offset smoker - heat problems

I converted my offset smoker to propane and love the ease of it.  The problem I am having is the wood I use for smoke often catches fire, making the heat hard to control.  I use an old cast iron pan for the wood chunks. I have tried moving the pan around the fire box and have put the pan on top of a couple of bricks, thinking that would insulate the chunks from the heat; but the chunks almost always catch fire.

I was thinking of buying an A-Maze-N-Smoker and putting that in the main part of the smoker and not using any chunks but not sure how that would work.

Any suggestions on how to keep the wood from catching fire would be greatly appreciated.

Thanks,

Dan