It appears that not only have I been vindicated but in fact my 18650 solution is used by Heng Long themselves.
A new M41A3 turned up today with a battery housing that is the same shape as a 6cell pack with a LiPo label.
When I split the cases it held a pair of 18650 cells.


Here's my solution...




And here is what Heng Long provide


They just put theirs in a familiar case.
Mine allows to swap the cells out and use a generic 18650 charger.
I rest my case.
Mine was $1.20 for the housing and as I use XT30 connectors that's another dollar and rescued cells from a tool pack that are in fact better rated than the HL ones.
So, to convert.
$2.20 and some rescued cells.
If you luck on a premium tool pack then you might even end up with a 2500mAh pack.
The generic cells that I'm using here are 2200mAh Samsung cells and have been perfect through 20-30 cycles so far so I can't see any reason why they won't meet their 1200 cycle duty as they came from a leaf blower with a burned out motor that had been used about 4 times and put on a shelf until the warranty had run out.
This pack gave me 10 cells so I could charge them all and drive my tank for over 2 1/2 hours and if I carried a charger, all day. You can charge at 4C so 20-30 minutes per pair of cells or even 4 at once if you have a reasonable charger.
Money for old rope.