Sometimes an outdoor field test just isn’t possible although I highly recommending taking every step possible to make it happen. In the case you are inside here is a training tip we wrote back when FasCat ran an indoor cycling studio in Boulder, CO over the winters:
I’ve just signed up for a plan and managed to do an outdoor test a couple of days early so added an extra indoor one on the Tuesday as prescribed in the plan.
Indoor FTP was about 15W lower which was better than expected.
Outdoor was much more enjoyable though