I did a bit more on this concept, a seatpost rack holding two EGO batteries. I like the 2.5AH packs (which I have two of), so that would be 5AH in parallel. I also have two chargers (at least) so could charge them in parallel.
The seatpost clamp design is not very accurate, those would be separately printed and bolted to the battery mount. Seatposts come in many sizes, so those would be printed specifically for the seatpost diameter in use.
The battery mount itself could use more design as well, I would like to support the contact blades on two sides like the EGO connectors do.