I burn my PT prints infrequently (except for my mistakes, which I literally burn in a 55 gallon drum). But, one trick I have used on occasion is to cut a matt board smaller than the inside of my contact printing frame. Usually, to burn a small area, I will cut it such that there is an inch of play on the matt board, side to side and up and down, when the matt is placed inside the contact print frame. I then cut a hole in the board slightly smaller than the area to be burned. I make the regular exposure, put the matt board with the hole in it in the frame and then set the timer for the burn time. I wear a glove and long sleeves and reach in to the pizza oven exposure unit and move the matt board around during the exposure. The board can only move so far (an inch) before it hits the side of the frame, so the hole stays over the area to be burned. I slowly, but continually, move the matt around during the burn time.