Hi,
Is there a 18650 to 2D sleeve?
a rolled piece of Paper is 
I used that in my 3*P7 4D MagLite and it works well.
I use tape. Its just a pain if you need to take it of.
corrugated cardboard is my spacer material of choice.
I think he wants 1x 18650 in place of 2x D cells in series.
So, not only added width, but also some 5 cm added length.
Bingo. 2D too heavy and I am putting the light in my car. Don’t trust alkie.
Sounds like a job for 3D printing
Or selfbuild from two pennies, a wire and spacer of the material you like most
I made one using plumbing PVC pipe to house the 18650 and coupling junctions. IIRC it was an ROP incan build that I sold off eons (almost a decade) ago.