A lot of people are able run the probes through the top vent hole. My smoker is homemade and that is not an option because of the short probe wires. I put my hole a little higher than the grate so I can start to feed the wires and they normally lay on the grate so I can reach them. I think if you put the hole below the grate your smoker temp probe will always be trying to fall through the grate. Just my $.02
Ya mine is homemade to and my probe wires are short. I got two bolts to put into smoker so that i can put them next to each other and just run the wires through the side and squeeze the little thing that usually mounts to the grate. So I'm thinking I'm going right below the grate.