Mit die quasi Dingsbums Maschine!
********
Without spending a fortune on an industrial temperature controlled cabinet, I would suggest placing a large cardboard box over the PC. The box would have a medium/large hole(s) near the bottom and one or two holes at the top. Then cover the holes at the top with a piece of cardboard.
What's needed is for the warm exit air to be trapped within the box so that the air drawn in to the PC is already warm. You would need a thermometer to measure the ambient in the cardboard box. If too cold... wait... if too hot... move the cardboard away from the hole at the top so warm air can escape.
This is a really clumsy pain-in-the-ass approach... but it should work. Basically get the PC to operate in a warm air environment and test.
The real concern I see is that the thermal throttling will start kicking in at around the 65-75C range. In your initial test on this cold winter day you were just around 60C on load... perhaps also becuase you tested with "open box" PC, or you have good cooling. In a closed PC, normal cooling, and warm environment, the thermal throttling could SCREW measured performance and gameplay.