I taped mine right to the flex on the bottom under the tub. My flex is one solid pc that runs across the bottoms of the tubs and then up the side to the next level/tub and so on and so forth. ( check Boaphile site for example )
I use 2" clear shipping tape and tape the probe right in the bend of the tape where it comes down the side then goes across the bottom.
I have my Helix set at 94 and w/ 4 sheets of unprinted newspaper on the warm end and two sheets on the cool end my temps stay 91-93 and 81-83 depending on the temp in my room and the weather outside ie cold or warm.