I don't have any direct experience with the wire style fixture, but my best guess would be that the heat being emitted by it is rising and escaping into the air above your enclosure.
As for the aluminum foil, I can only relate it to cooking (that's what I do for a living) and guess that it would direct the heat downward into the enclosure.
I would probably use a dome if you have one that supports the CHE rather than wrap it in foil, though.
You're not doing anything wrong, you're just getting through the dialing-it-in phase. Once you get it all figured out all you need to do is monitor it daily and you have less stress for you and your snake. It takes a little time, trial and error, but you'll get it. Many people, myself included, aren't properly prepared before taking our first snake home and kinda have to learn on the fly. It's a learning experience. that I went through once upon a time. I now make sure my enclosure is dialed in and my husbandry is solid for prior to bringing any animals home. Live and learn.
I wish I had a more definitive answer for your current question, but I figured my two cents wouldn't hurt. Hopefully you'll get an answer from somebody with direct knowledge on the topic soon.
Good luck!
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