The battery is Varta poliflex - I've found some models overviews here:
http://www.matrix.es/docs/PRODUCT_OV...TTERIES_en.pdf
There is an interesting battery (2nd page) - it seems to be the same model but doubled - so that it is twice as thick. I wonder if there's enough space to squeeze it in? kleykenb can you check whether it is the case?
Alternatively there's another one twise as wide