I would bet the battery cover for the Seidio 3500 battery would work. Might just need at most a thin shim between the 2600 battery and inside of 3500 cover.
I have the 2600 battery and it is great but it takes a long time to charge back up, even in my external charger. I have determined that I want to charge my batteries via the external charger from now on. Two reasons.... one, it will be less wear n tear on the charge port of the phone. Second, it eliminates the need to bump charge.
Since I am currently at 37 hours on this 2600 battery and setting at 39%, I think it will work out great to just swap batteries every other day thru the week. Problem is I only have the one 2600 battery. I could easy pick up another 2600 from E Bay, but I am none too thrilled about the form of the cover that come with it. Makes the phone look like a brick. Although the flat side is nice for holding the phone up for watching movies. LOL
But there are no cases that can even be modded to work with this brick cover. The 3500 cover allows for modding. The back hump could be cut out of the silicone case I got from VZW. With it on, at least there is protection for the perimeter and front face.