Chips burning instead of smoking?

Well I know I need a good off set smoker or WSM or UDS but for now all I have is the gas smoker. Been having a problem with the wood chips just catching on fire and not smoldering very well. I always soak them but that does not seem to help. Once they dry out they light up. Basically have two sliding drawers that you pull out and fill about 18" long by 2.5" wide and an inch high. Anyone have any ideas on how to get the wood smoking better? Was thinking about laying a plate about the length and width on top of the wood. Might keep it from getting enough oxygen to catch fire. I usually don't have the problem if I use large chunks instead of chips. Just not easy to find Chunk smoker wood in my area other than Mesquite or Hickory.
David

Heres the smoker, you can see the two pull out trays below the door. They sit above the burners and get too hot to smolder correctly. Like I said not much of a problem if you have chunks and not chips.