got my 3500mah battery yesterday in the post.
Yes it is double the thickness so if I flip the battery so the contacts line up, they don't actually touch the contacts on the phone. However this hasn't deterred me from carefully dismantling the battery and make a new battery housing with the contacts in the correct position.
As for the battery cover, well, I got my spare Wildfire S battery cover, and gradually trimmed a battery sized hole in the back. I have got plans for making it smoother and more streamlined but it's just ready at the moment.
End result: HTC Wildfire S with at least 2.5x the original battery capacity (picture attached.
Cost me all of £6 (with free P&P), about 5 minutes cutting time, and a partially sliced finger where the blade slipped......