I have always used my planks, after soaking for the stipulated time (at least an hour and longer is better they say) direct. I use Kingsford Original, spread one chimney of lit evenly over the charcoal grate of my OTG. I pre-heat the plank (the side that the meal touches for two -three minutes to sterilize it, then flip and heat until it is smoking then load the food (mostly fish but have also done pork chops, whole onions, etc.). I can re-use the commercial planks two-four times. When they give up the ghost, I break them up for smoke wood for the OTG.
Not to be argumentative, but I don't see how you get smoke without charring the plank from direct heat. I have been MORE than pleased with the results I get on a variety of fish, chops, and veggies. So have my guests...
I have only used commercial planks (generally bought on close outs in quantity) but my son, and avid griller went to Lowes and bought an untreated cedar board 8 feet long and cut to size. It is billed as a 8"x1"x8' and he told me it cost him about $8.00. That is a far cry from come commercial planks that sell for upward of $7.00 each for a 3/8"x1"x14". Further, because of it's thickness, it can be used MANY times. He has had several cooks on the first plank he cut from the board.
As long as you make certain that they are untreated boards, that would seem to be the way to go.
I have digressed - I always cook at medium direct on my 22.5" OTG with a plank.
FWIW
Dale53