I have made a a cardboard model of Varta battery that I have found. Only way it can fit is by taking of the rear fender and unbolting the rear sub-frame. And still the height can be an issue because of the seat pan. So it will need some expert adjustments.
For now I am not willing to take these steps, and original Banner 30Ah will be available at the end of the month.
187mm 127mm x 227mm.