Delta is your only solution when using variables. Sorry to be this blunt.
I say use the A/C set to lower temp that you can keep, then do testing. (I test everything at 21.5 to 22.5*C) (humidity can be a bitch too ;)
Also, while you are in a mindset of many others, you do throw us under the bus...
I believe this is where "deltas" come into play.
A few sites will not use actual thermal results due to the same issues you are running into. While it may not be as perfect as you want, it is the closest you are going to get without removing the variables.
The point is you are worrying over something that will cause only a few degrees of temperature fluctuation under a cooler with strong mounting hardware.
Back in the day (775 - 115x) we used to lap IHSs down to 2000+grit, and at the end of my experiences, I got 3 degrees improvement.
I mean by...
While I apprecite the work and effort here, one thing is being left out of these tests.
Proper mounting pressure.
While I am sure you are pressing hard in your glass tests, CPU coolers exert a ton of force onto the IHS, and could likely change what you are seeing in your way of testing.
+1 for Anker. I just got one with plenty of reserve and came with a flip out charging plug (less cables). Wasn't that expensive compared to others either.
A drill and bondo are your friends here.
You should be able to drill the weld, releasing all parts of the latch, but then you will have some body work to do to make it all smooth again.