Regardless of what anyone else says:
I use foil tape to tape my probe to the back of the shelf on the flex-a-watt.
Then again I have a tub set up with a thermometer in it that I rotate through the rack every couple weeks to make sure that all my shelves are heating.
I adjust the thermostat to in tub temps according to the thermometer tub
All my racks are home made.

Your AP rack should have one long strand of flex-a-watt weaved through it, right?
Can you put the probe on the side of the tub where it goes up to the next shelf?