The before mentioned batteries fit as you can see in the posted pics. the batteries are the ones laying in front or back of the tornado. There are NO modification requirements to make these batteries fit inside the compartment, the only modification was the connector for the batteries.
Since these are 4500MAH 6s 50C only two (2) will fit in the cavity. The amps do not change upwards, you put 2 4500Mah hour batteries, you get 4500Mah worth of power, vs 4000 Mah per battery X3 with 8C from Yuneec. There is a clip on you tube in German that covers the use of after market batteries for the YuneecH920. The pilot in the video confirms that the use of aftermarket batteries are legitimately usable.
Here is the you tube link to the video; (German only)
www.youtube.com/watch?v=CbbgRRCWgFQ
www.youtube.com/watch?v=ZAK3Es21wRI