It would be helpful to know what type of gas grill you are using?
I have a weber genesis silver B, I also have the smoking accessory that uses a smoke box on the crossover gas tube, and a cut down grate so you can open the box and feed in more chips without removing a grate.
I have tried several different things and the best and really easy thing...
My weber has flavorizer bars... I take a big chunk 2" x 3" around that size, and drop it between the flavorizer bar and the inside wall of the weber. In my weber the front flavorizer and the back flavorizer has only about 1/8" gap between the wall and flavorizer at the bottom of the V. If I'm not using high heat the wood just smolders there for a couple of hours. If I'm using high heat, I just fold up some alum foil and push it down in the V it slows the burning process down.