Hi MD55, not easily penetrable is exactly that. Best thing to do is contact your RFO (regional firearms officer) and ask them as they will be the person conducting any inspections
you may have. I know people who have used the cheap thin steel tool boxes from supercheap, but now the RFO's are really trying to tighten the belt on those.
As far as wood is concerned unless it is a thick hardwood I wouldn't even entertain the idea, just my opinion though.
Personally i would be using the safe for the firearms and maybe doing a mock up on the external of the safe with whatever wood you like, as the internal
storage will be what is complying with the legislation.
I hope that helps,