Wotcher, I fitted 4800 mAH saddles to mine, pics on my flickr page in the radio controlled car set;
http://www.flickr.com/photos/tarcus/sets/
They fit under the top deck fairly tightly and I used velcro fixed in the middle with some old servo horns to act as stops and as anchors for the velcro. If I had my time again I'd probably use something different for the central fixing as I had to drill 4 holes right in the centre, I'm hoping that won't weaken an area of fairly thin chassis material too much.
It's a little bit of a bodge, this is my first RC car build since I was 14 so I don't think I did too bad..