I have just measured the current draw of my Lidl (branded Powerfix) calipers.
Switched off - 8.5μA, switched on 9.5μA.
So my 30p LR44 should last a year or so and I should stop worrying about turning it off.
Not Scottish, but Tim Lane http://www.trlanefabs.co.uk/ABOUT-US.html made me a nice alloy tank a few years ago, exactly to my drawing and delivered in a few days.
Is that a Prestolite?
Put your multimeter between the battery negative terminal and the alternator case while charging to check if there is excessive voltage drop in the negative circuit.
If you need to add a connection direct to the alternator case you could put a ring crimp under one of the...
I bought mine from here: http://www.mailspeedmarine.com/life-jackets/ocean-safety/kru-sport-iso-lifejacket-auto-harness-with-light.bhtml, with light included. Harness version has integral sprayhood too.
Only used for a couple of trips in the dinghy so far, but seems comfortable and quick to don...