Modified the mod a little bit. If you use a slightly longer tether, you can attach it to the stock.
Place one knotted end of the 550 cord through the battery cap as before, but leave a lot more length than you will think you will need, and don't tie the other end yet.
Remove the battery compartments and you will find a small notch on the inside. Route the untied length of tether through that notch to the rear, and pass it through one of the lightening holes in the spine of the stock. Secure the battery compartment by holding it in place. Then route the tether back forward through the notch on the other battery compartment and snug it back along the stock body. Insert the screws and nuts and tighten slightly. Play with the length of the cord so that the battery cap that is already secured has enough slack to be pulled out and inserted without difficulty. You just want enough tether length to allow you to remove the cap and have it hang freely, you don't want 6" of cord hanging around. Once you've got the tether length comfortable tighten the screws to where you want them.
With the untied length hanging out forward, thread it into the battery cap as before. Measure out a similar length to the other secured end, and remember to make it a little longer so that you will have enough left to knot inside the cap. Cut the cord, burn, and knot.
Even with the tubes tightened, there is still some play in the cord and you can even it up if you're worried about it. The extra slack in the tether can be tucked back into the voids of the stock underneath, and the levers on the caps will help hold it down.