I give you, my latest defence system:
~=Magma
.=Obsidan Floor (natural)
#=Solid Obsidan
S=Support
O=To Outside
F=To Fortress
_=Empty Space
z-1
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####################
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####################
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z0
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O..................F
O..................F
O..................F
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z+1
____________________
_##################_
_##################_
_##################S
_##################_
____________________
____________________
____________________
_##################_
_##################_
_##################S
_##################_
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As an extra bonus, when the blocks of obsidan are dropped, it also opens some floodgates on higher levels that allow water to fall into the magma, to keep the miners happy as the rebuild it. I'm still working on a version that can be used several times between reloads, possibly tall enough to allow me to keep up a constant wall of magma mist.
I'm still waiting for an elven caravan to come along to test it.