I don't know of any readily available at the parts store long 12v batteries that would fit the bottom tray designed around the group 2E battery which is around 19 1/2 x 4 1/4. There are commercial batteries about the same length but are much wider.
An interesting approach by a poster at PAC using two 6v Optima batteries which should be readily available everywhere would work.
http://www.packardclub.org/forum/viewtopic.php?f=15&t=2251&start=380#p18945 He built a custom hold down which mounts to the original tray to hold two Optimas sitting on the tray. While he is running his batteries in parallel keeping 6v, he does mention the option of placing them in series for 12v. If you wanted to carry the disguise farther some black textured ABS or other suitable sheet plastic could be formed to mimic the square case of the long battery and hide the round battery sides.
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