It apears that you are a victim of both Mercedes and Interstate Battery. Mercedes should have given you an extra inch of cable, and Interstate ought to take the fool thing back if it won't fit.
This may be evidence that the dolt who designed the 82 Buick Regal I once owned with the rear windows that won't roll down, or the razor-sharp electric window fittings in same is still alive and annoying innocent motorists.
I have an Interstate battery in each of my cars (the 123 and the 124), and although there is just barely enough cable to make the battery (with disconnect) fit, it does fit.
I observe that in the 123 (300TD) there is some sort of fuse that was screwed to the body whose leads have been cut. Perhaps this was to make the battery fit. I am assuming that since everything works, this item has been re-fused.
It would appear that you could either make a very short cable (perhaps a trip to a junkyard would make this cheaper) or perhaps epoxy the plywood to the base of the battery with JB weld or some other brand, and worry about how to unglue it later, or just toss it when its time comes. If the plywood will stick under the battery holders, this should work.
Also, they sell battery holddowns that might work. You would have to drill a couple of holes in the body, but in some manner you could hook, bolt or screw the battery down. Perhaps this could be a job for Mr Bungie, or Mrs Bungie as well. I have been known to successfully hold down a bouncing battery with a bungie in a lesser vehicle.
Since you have two batteries, here is a thought, perhaps a dangerus thought, so I am hereby disclaiming all blame. You have two betteries: one fits, and a newer one that doesn't. A battery that is not in use will die unless you put a trickle charge on it, and even then, the acid will slowly degrade the plates. One way that the acid cannot harm the plates is to remove it from the battery and store it elsewhere (the thickest plastic you can find, or glass that you can put where it cannot be broken).
You probably won't be able to remove all the acid, but you can probably collect most of it in a sturdty plastic paint roller tray and then pour it into a suitable container in with a plastic lid or perhaps a plastic cover and a metal lid. Then you can rinse ot the battery and perhaps dry it out by letting it sit in the sun or use a hairdryer on it., and store it away as well.
When the first battery has sparked its last, then you simply refill the newer battery with the acid, and you have a new battery. Charge it up and you're good to go. Be REALLY CAREFUL when mixing acid and water. There are rules about which you can add to which and avoid an explosion and subsequent maiming you can look up. If you need more acid, they sell it in auto parts stores, or used to. I am assuming they still do.
Basttery acid likes to eat holes in things. Wear the clothes you hate they gave you three Christmases ago: the polo shirt without the pockets, or perhaps a leisure suit or disco finery.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"