In first tought, fixing from bottom was OK, but I am realy worry about chassis flex and weakness. If I use guide posts I need at least two locking mechanisms, And locking from bottom looks risky too for me. Why not use simple way one side of box going behind of chassis (difficult explain) like usualy battery trays in different devices and second side have locking mechanism and maybe one or two guides with integrated locking mechanism. But dont have nice ideas about locking yet.
Now I think inserting battery from side is bit better. My english is bad and difficult to explain but I drawed what I mean.
On the picture Green is the box and red same box but how I mount it. On topyou can see how box fix to the chassis. Box sides slips to the socket.
http://www.hot.ee/tom255/design.JPG