Tube shaped internal battery mods are rarer than box mods because internal battery mods generally use LiPOs which can be any shape. It’s the replaceable battery mods that are limited to tube shapes. That one is particularly tiny making things yet more difficult. If things weren’t limited to tubes I would have several suggestions.
The problem that happens then though with external battery mods is the industry has settled on the 18650 form factor and 18500s and 18350s have become hard to find.
The suggestion of the EHpro is a good one. Another is the nunchaku. It’s more bulbous though. I can’t vouch for length with those.
If you’re just interested in really small devices there is a whole thread
Little itty bitty mods under $100.