Alright, if you would like to. The battery I bought is non-brand, but lasts longer than the 1200 mAh that comes with the slim. It's an 1800 mAh battery, and you will need a bigger battery cover.
I have a spare bigger battery cover, but it's black. So if you have a silver, the back might stand out a little bit. If you would like, I would happily include it in the shipping to you. That way you have something to work with until you can get a matching cover or paint it or whatever.
I just looked on eBay and they have the bigger battery covers, for about 3 dollars and 50 cents.
Maybe I could buy it for you if I have enough.