I remember someone once posted the diameter needed to achieve 1x18650, 2x18650, 3x18650, etc., but I can’t find it. Does anyone still have this info?
For unprotected 18650's that are 18mm in diameter, the following minimum ID would be needed to achieve zero clearance.
| # of Cells | Multiplier | ID Needed |
| 1 | 1.00 | 18.000 |
| 2 | 2.00 | 36.000 |
| 3 | 2.16 | 38.790 |
| 4 | 2.42 | 43.452 |
So a tad bit more space should probably be provided. For protected or cells that are larger than 18mm, just multiply the actual diamiter times the appropriate multiplier in the above table.
EDIT: Added more info and rounded to 2 digits (rounded up in each case).
Have a play here 
Cheers David