As much as I would love to say the mora or typical 3 to 4" scandi style knife, if you're talking about shelter building and firewood without the luxury of an axe or saw, I would definately go for a heavier, longer knife than can chop effectively. As has been said, a machete/golok/kukri would be the best option IMO.
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Has anyone tried to build a lean-to shelter with reflector and collect enough firewood for cooking and staying warm overnight, using just a mora? I think I'd die of exhaustion before it got dark