Thanks for the thought - I have thought of that and may end up going that route soon if fabbing a tray is too much trouble. In the end, I probably don't NEED two batteries but do like the idea for safety sake way out in the wild.
G
A PC680 could be installed where the jack used to be as a backup starting battery. I've jump started my truck a number of times on a PC625 and it easily did the job. This approach simplifies the battery management. All you need is a solenoid to connect the backup battery when the engine is running. When you need to use the aux battery as a starter, just use some short jumper wires.
Alternatively, I wonder if a similar arrangement could be made with the smaller battery permanently wired to drive the starter exclusively. Then if the small one dies, use jumpers again to the big battery.
Either way, what I have found is a small Odyssey will easily crank a big engine, it's all you really need for starting duty. It's got the amps, just not the capacity for "house" loads.