what about this http://tinyurl.com/5olbh6 (about 5 euro) + 8 AA uniross hybrio (about 19 euro) and some diods to lower the voltage and save against the inversion recharge (many battery pack, if discharged, are recharged by the device you want to recharge !) ?
so:
2100*8=16800*1.2=20160/5.4 =about 3700mAh of capacity ?
seems not too difficult to made....

all for about 25euro.

sorry for bad english ;-)