Epoxy has a large thermal expansivity probably not the thing to use for low temperature potting -
silicone rubber (beware the acid curing sort though) is probably available that won't apply any
stresses across the whole temperature range, being compliant.
Mil spec components are usually ceramic to specifically avoid the epoxy thermal expansion problem,
which can damage gold bondwires for instance in chip packaging. Most ceramics have a low expansion
coefficient similar to silicon (alas gold itself is rather higher).
I'm not sure insulation is useful, keeping the temperature constant (ie close to ambient) is going to
reduce cycling and give the highest reliability I think (assuming the device is intermittently powered).
Diurnal variations are presumably not an issue in the high artic?