I'm using the Lugu Lake stick-on receiver, so it doesn't matter how thick the back is.
But remember, Qi charging is about 600mA, so if you have a 6,000mAH battery, it's going to take about 12 hours to charge it wirelessly from 0 (which you never do [you try not to discharge a lithium battery to less than 40% capacity], that's just how you compare charge rates.)
Two stock batteries (IOW, buying one more) and an external charger (which charges both the battery in the phone and the one out of the phone at the same time) will give you twice the capacity, the same charge time and less than the price of a larger battery and cover (about $20 for a charger and battery).