Nice comfy padded floor is essential and a thick curtain or insulated swing door to keep out the wind.

You can also look up winter pet shelter plans online with details on wind break walls, etc

Speaking as a Fellow Moose Jockey, Depending on where you live you will need some form of heat source. Even if its radiant heat from your house could be enough in milder temps. (Build it up against the house if possible)

A basic wood shelter would be ok for "semi-cold" temps but once you drop below -10°C its going to be difficult for them to stay warm without a heat source. Even lining the walls / floors / ceiling with multi layers of old blankets will be better than nothing

Just Insulate it the best you can - wood wont be enough come -20°C - on those/any cold days, bring them inside plz.