the black/grey is standard kind,
the yellow some special kind, extra thin. I think something like "shrinking rate" 1:3 was the special there.
Got both at a local Shop for electronic parts.
Try to get one, that just barely fits over. something like 26 mm inside diameter, or so. That will not get too thick when shrinked.
and try with original shrink of the cells removed - watch not to tilt the stick: shorts!!! -
put one strip of adhesive tape alongside over the whole stick to isolate cell metal from negative contact.
shrink it
PS: since I modded my 6P to 18650, Flupic driver and an XR-E P4, some 3 years ago, my 8X and 9N became shelf queens
(instead of the 6P which had that job b4 because of the expensive, non-rechargeable batteries).
Sure, the 9N is the best incan ever produced - extreme size/output/value ratio, rechargeable, two outputs - but ...
Even with these 1 hour 25 mins battery runtime sticks (for the 9N), they are not equal.
(and in short the 6P will house an XP-G)[