I've been lucky to be able to accumulate a lot of good hunting clothes over the years but I have found that no matter what I wear once the temperature gets down into the 'teens or there is s stiff wind, it's only a matter of time until I'm cold no matter what I wear.
The key to staying warm for me is not just sitting there waiting to get cold. For one thing, I stand almost all the time. Even though I have had back surgery and am dealing with more degenerated discs, standing uses more muscle than sitting and generates enough body heat to make a big difference over sitting. Sitting also compresses your layers where standing allows them to maintain their loft better. Standing offers much better visibility too, I get surprised a lot less often when standing.
Isometric exercises of the big muscle groups will definitely keep you warm with little if any movement. Full body clenches, "most muscle" hero clenches, lots of others can be done and there's really no movement at all, but lots of body heat generated.
A very slow partial squat, 6" or so, then hold as long as you can, back up. Any of these little exercises will help keep you warm on stand. Best to start them before you are shivering though.