I have found these and fit the battery box in a pair, any thoughts?
Dualsky ECO-S LiPo Battery, 5200mAh 6S 25c
Capacity (mAh) 5200
Dimensions (L x W x T)152 x 45 x 41 mm
Max Cont. Discharge25C
Max Cont. Current (A)130
Max Cont. Output (W)2886
Max Charge Rate 4C
15% more amperage than the 4500 mAh I've tested, but also 30% more weight. Not sure if this will translate in longer flight times...
I see this brand is quite popular in Australia, price I've found is around 165 AUD = 100 EUR = 110 USD per battery.
For that price you get 2 x 4500 mAh batteries ;-)