Jeff at Boaphile recommends placing the probe thru the back in the hole that the flex cord comes out of and put it all the way over to the right or left so it won't interfer w/ sliding the drawer in and out. He said place the probe right in the corner where the flex bends 90 degress and secure it w/ clear plastic shipping tape. I spoke with him a few weeks ago as I ordered the 4 tub ball room rack from him.
He also said you very well may not need to drill holes. It's not required but he said if temps run to hot or humidity runs to high to drill or burn them in the sides not the front or back.
Hope this helps