Keep in mind my case is the Cooler Master HAF 932, I also probably have more airflow than the average case.
If you splash out the
$11 + fees for 3DMark, you can set it to a continuous loop of one Fire Strike test that will heat you up real good. 3DMark is a much more stressful benchmark than Heaven and you'll find your max clocks much faster.
Alternatively, for free you can download
FFXIV Heavensward benchmark which gets your GPU hotter than anything else you can use basically, it does have loading times between segments but not even real games can make the GPU hotter. It's by far the best pure GPU stability test I know of.