Much appreciated @Cyborgium!
A friend printed this out for me including the top without a fan opening.
Fitment is superb.
For others out there, I flipped over my RPi fan to pull cool air unto the CPU then stepped up from 3V to 5V with a generic 40x40x12mm heatsink. This yields idle temps around 37-40C and active temps in the 45-55C range. Running benchmarks shows a max of 67.5C at 100% utilisation across all cores (no thermal throttling). Before the above changes, temps were about 10C higher for each condition.
