Here is how I have put the batteries either side of the motor using some balsa wood, doesn't look the best but it does the job. Now I have loads of scope to move the batteries back and forth for the best cog. I haven't tried it out yet but in the bath it sits in the water very evenly. My last p1 was rear heavy with the batteries at the back.
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I am here: http://maps.google.com/maps?ll=52.472140,-2.153722