Similar to Rob's suggestion. Third battery under the starb settee, wired in parallel with one of the two batteries in the port lazarette, to Bank #2 on the master switch. Both Bank #2 batteries are starter/deep cycle hybrids, so I can use Bank #2 for both house and starting. Bank #1 is a single purely cranking battery reserved for 'emergency' starting. So I've got two ways to get the engine going. I don't have much in the way of appliances, and I'm mostly on shore power, so this works out. Not recommended for everyone.
On the E35-2, I think the third (and fourth?) battery would be better under the (port) quaterberth. Haven't measured but I think there is room. Alternatively, near the other two batteries in the starb. lazarette which would be a shorter cable run to the master switch (but Indigo lists to starb., so I try not to put weight there).
--Steve