Just an item for consideration...I thought of all sorts of places to mount mine, but ended up with it on the fuel pump cover *rather* than on the cross-member behind the seats or some other location aft because anything back there in the baggage area is essentially inaccessible in flight, so you might be able to reach the extinguisher and unlatch it back there, but *if you drop it*, you may not be able to get to it (and in the heat of an emergency, that scenario is not too far-fetched).
It wouldn't be easy to get if if fell down by your feet, either, but it'd be doable (pitch up and let it slide/roll back).
Just something to consider.