I recently purchased 4 protected NCR18650B, these are like the standard cell 18mm wide and 650mm long, but have a battery protection circuit built in to prevent over charging and discharging.

I purchased them from: https://it.aliexpress.com/item/new-4-pcs-original-protected-18650-rechargeable-battery-3-7-V-3400-MAH-battery-for/32654934119.html

The addition of this protection add 0.2mm to the width and 3.6mm to the length, making the overall size 18.2mm x 68.6mm.

I also used some cheap terminals for the end connectors.
https://www.banggood.com/10-Pairs-Silver-Tone-Metal-Battery-Spring-Plate-Set-for-AA-AAA-Batteries-p-1046111.html

I decided the bend the terminal by 90 degrees this ensured they were fixed into one location.

Drawing for all the assembled parts are included.
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