A club member built the maggie m last year, and although tight he can squeeze a 12v 7amp lead acid battery through the wheel house hach, it may be worth looking at the smaller 12v 3.3amp lead acid batterys if your concerned about hatch.
The 3amp versions are almost half the depth of the 7's and i see no reason to not use two hooked up in parallel to bump your amps to 6.6 for a longer run time, a solo 3.3 battery will run one of the mobile marine T12's for several hours, a torpedo 850 for about an hour at full blast, or 2 of the mfa geared motors for around 3 hours consistently (i use these in a lot of my heavy weight models so speaking from direct experience)
You can get the 12v 3amp LA's from the component shop linked here ...
http://www.componentshop.co.uk/12v-3-3ah-sealed-lead-acid-battery.html
RR
_________________
I'm Not Anti-Social, I'm Anti-Stupid.