We are happy to finally have a small inventory of Di2 battery holders for select 25.4 seat posts. We have two kinds, a rubber one and a plastic one. The rubber one will protect the connector side of the battery best.
The plastic one is less expensive and slightly more tolerant to internal dimension manufacturing variation.
These holders are intended for specific seat posts and have been tested on each type. If the seat post has the exact same internal diameter size, it should work.
If you have another size, and are sure of the exact measurement (you used calipers), we can make a model specific to your seat post. At present, we only make plastic style custom sizes.