You can thank Gabby for the eye rolling dad pun of "Ras brrrry pi", although very fitting in this case. Speaking of fitting in cases, let me stop with the puns, and get to my findings.

Hot Takes

Problem/not problem: I own a couple of Raspberry Pis and I want to insure they stay chilly under load. What is too hot? Here's what I found...

The official operating temperature limit is 85°C, and as a result the Raspberry Pi should start to thermally throttle performance around 82°C. 

source: https://makezine.com/2016/03/02/raspberry-pi-3-not-halt-catch-fire/

...also...

The BCM2835 was designed to operate within a closed case without any problems.

source: https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=13080

Good, now I know what temps it stay away from. There are a bunch of ways to display the CPU temp, but the application I run on my RPi3 reports the temps in a nice web GUI. From what I see, my idle temp is 45°C, and my max temp so far has been 75°C. Good, now I know what temps I'm working with.

How the Jones Fared

Most tests I've found on the Internet have been passive heat sinks vs 45mm case fans, inside and outside the case. Which is not my particular use case. I would like to use my Short Crust case, without hacking it all to bits. Sure, if I submerge the RPi3 in a vat of circulating mineral oil, I can keep the temps in the 60°s but, I feel somehow this will eventually relocate itself all over my floors. I'm also not in the market of propping large fans against my RPi3. All seem to be proven solutions, but not the prettiest. I would like to add cooling without the hassle.

Eenie Meenie Miney Mo

I starting looking into heat sinks. Copper vs aluminium, fins vs pins. Even if it's worth looking into larger sinks vs tiny ones. It seemed as if it all came down to price. In my searches, I've seen heat sinks from 30¢ up to $25. There's a solution for every budget. Then again, the Raspberry Pi was made to fit in anyones budget.

Passively Looking, Actively Searching

So, in looking for the largest heat sink that would fit inside my case, I ran across the dual fan heatsink for the Raspberry Pi. Since I'll be inclosing the heatsink, it only make sense that active cooling is the way to go. Active meaning, forcing air across the fins. Now that I'm looking into fans, I'm now worried about fan noise. That's right, fan noise from a 20mm fan. I can assure you, inside a case, from across the room, a 20mm fan isn't going to be heard... in a distracting manner.

I've build every computer I've own, minus mobile devices and laptops, so I know my way around a motherboard. The thermal adhesive tape that was send with the dual fans reminded me more of sound deadening material than anything else. It's pretty thick, and feels a tad dense. I didn't want to risk using something so thick on top of a CPU, so I opted for good ol' tried and true thermal paste. (Arctic Silver 5 to be precise)

TEST 1: Not Really a Test

I had recently done a fresh install meaning, my RPi3 was out of the case. So, sitting idle and naked, the temp was 45°C. With no thermal paste, because I forgot to click order the day before on Amazon, I simply pugged in the dual fans and genteelly placed it on top of the CPU. I checked 12 hours later, the CPU was at 39°C.

TEST 2: In the Case, No Fans

Placing the RPi3 back into it's case and letting it live in its environment for a bit of time, the temps are at 52.5°C. Well within range of operational, maybe slightly high for idling in my book.

TEST 3: In the Case, Fans, No Paste

Sometimes you just don't have thermal paste laying around, or, you don't remember where you put it. Since the RPi3 is made to run fanless, it's not going to hurt just placing the fans on top. Baring the creation of any short you may cause by doing do. I risked it. After a while, I checked back in on the CPU temps and it was at an idle temp of 51°C.

TEST 4: In the Case, Fans, Paste

OK, final test, how I expected to assemble everything. Thermal pasted and enclosed in a case. There's not a ton of airflow in the Short Crust case but, it's definitely not air tight. After a day, the RPi3 is steady holding at 47°C.

One thing to note, during all test the ambiant room tempature was between 72 to 74°F (22 to 23°C). I also know these are idle temps, and a better test would be under stress. My RPi3s don't get maxed out too often, so I felt just checking idle temps was good enough.

Pi Felicia

In the end, I wasn't looking for the coldest, or cheapest solution. I wanted something simple to install, and would work in such a tight space with zero airflow. This is buy no means the best way to cool a Raspberry Pi, but I think the dual fans solved my problem/not problem just fine. All for about $10. Plus, I'm now using the GPIO pins for something. Is it overkill? Maybe, maybe not. What if I was thinking about adding a thermal speed controller?