daddle, I can think of three or four reasons for going with larger wire size inside the battery box
than that of the wire leading away from the battery box
One would be in case you ever want to upgrade the wiring
outside the box. Another would be if you ever want to use the house bank to jump the engine
starter battery. A third would be that you want to reduce the internal resistance of your battery bank as much as possible to supply the best voltage that you can. Finally, these wires are not fused and while you wouldn't expect currents larger than those drawn from the bank, if a cell ever shorts out or otherwise becomes unequal to those of the other batteries in the bank, there could be some high internal currents.
I agree with you though that, although the batteries should be secure inside the battery box, a flexible interconnection would be better than rigid bars. The box allows a bit of room for the batteries to expand and contract
with temperature, etc..., so there is, literally, some wiggle room.
witzgall, that's my plan -- connect to the bank at opposite corners, as it were.
Bill, I've been through the calculation a few times, and I believe the #6 to be good. It's a simple boat, with not much more than a few lights and some sailing instruments to power
Thanks all for the input!