Improvised/Non-Proficient Weaponry?

If I’ve got a Mage who picks up the Fighter’s battleaxe, or an unarmed Cleric/Crusader picks up a chair to brain a goblin with, how do I judge the result of the attack throw? Surely it doesn’t automatically miss – but do I ask for a modified 10+, or impose a -4 penalty to the usual target, or something else? I’m comfortable winging it, but curious if there’s an official answer I’m missing.

Looks like they attack as 0th level characters which from this chart
means they hit on a modified 11+

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