I suspect the adhesion problem is due to his fur. You said you "clip.shave" the spot, which implies there is still some fur under the patch. Removing it would be like trying to remove duct tape from a scalp full of hair. You may need to reshave the spot so that no fur is remaining each time you replace the patch.
Fentanyl is an *extremely* potent medication -- those patches are not just bandaids with goop in them, they are designed using *HIGHLY* technical specifications -- the microscopic nano-channels in the membrane allow an absolutely PRECISE measured amount of medication to be released onto the skin to be absorbed on a specific schedule.
Altering or changing the design of those patches is out of the question -- especially since, nowadays, the actual ADHESIVE contains the medication. Fentanyl is so-many-times more powerful than the most powerful narcotics, not something to be messed with (or even TOUCHED -- you wear gloves, don't you?).
I'm not trying to scare you... well, actually I am, a little bit -- I'm just trying to make you aware of how serious this medication is, and to convey the importance of talking to your cat's veterinarian and/or pharmacist about this matter.
If it were anything else (e.g. a bandaid with antibiotic in it, or whatever), that would be a different story. But, no, this is serious stuff. Please heed my post.