I too have looked into how to carry a spare gas can.  I will probably go
on the back, since I am concerned about heat and leaking on top.

In any case, I would highly recommend NOT FILLING THE CAN WHILE ON/IN
THE VEHICLE.  You may have read all of the stories about truck beds
(static electricity builds up and ignites the fuel as you are filling -
boommm!).  I would imagine that something similar could happen on a
westy top since it is fiber glass, or also in other areas of the
vehicle.

Long story short, it should always be filled on the ground next to the
pump.

David