Here's the information you need Steve. One of our members used a US Batteryspace 24 volt 10Ah NiMh pack to recell a Lafree Twist battery and found the same as me with the Powacycle packs, that a small amount of the surrounding packing needed to be removed to squeeze it in.
The published dimensions of that Batteryspace pack are:
9 1/4 "L x 3 3/4 "W x 2.5 "H
and you can see it's appearance here:
Batteryspace 24 volt 10 Ah hour pack
Since a little of the surrounding sleeving has to be removed, it would be best to deduct a minimum of one sixteenth of an inch from each dimension. I've done that and converted to metric for convenience, so the maximum dimensions for Ping to work to are :
234mm L x 94mm W x 62mm H
Since the pack and box area are not exactly rectangular, you may prefer to deduct a further 2 mm from the length dimension to ensure the Ping pack fits if it uses rectangular components,
When the pack is in the Lafree box it leaves a smaller irregular area at the foot which takes the fuses and connections. You can see that pointed to by 2 and 3 in this photo from my Lafree Website:
