Marketplace
Dox and VAUX battery and speaker units for Amazon echo dot 2, ranging from $59 rrp + down to $11 plus delivery.
Dox in grey $11.00.
VAUX in carbon $19.00.
I purchased the VAUX and it sounds as good as anything UE or Sony do in that small portable size.
It took me a while to find this information. Sharing it here to save you time.
VAUX
Sadly, VAUX does not have a wireless charging base like the DOX, and the DOX is not an external speaker for the Dot.