I just mounted an 80mm res to my 900d last night. I mounted it to the drive cages on the right side, but this requires installing the drive cage to the middle area in order to have the screw holes for proper positioning. You'll also need to buy some small washers, spacers, and nuts.
Once you have the cages installed, then you use four long screws + spacers together and screw the reservoir bracket into that. Not all four of the bracket holes will line up with a hole on the drive cages though, so you will need to place screws wherever there are holes, and where there aren't, there'll at least be an opening that leads into the drive cage. With this, you'll need to put the screw through the bracket, through the spacer, and then into the drive cage opening. On the other side of the screw, you then put a washer + the nut around the screw and tighten. This will hold the screws in place and allow you to mount everything properly without having to make any new holes. However, the space for tightening the nut on the inside of the drive cage is pretty difficult to do, especially if you have a drive mounted to that sled (as I do), because you'll need to keep the sled in the cage while you screw everything in, as the screws will prevent the sled from coming out once installed. If you want to ever remove the sled later, you'll need to unscrew the reservoir bracket first.
So if you have a drive installed on the sled, then your hands will likely not be able to fit in there to hold the nut in place while you tighten. If this is your case (as it was mine), then you'll need to have some small grasping tool (like tweezers) in order to tighten. It's a fairly tedious process, so be prepared for a little frustration. Here are a couple pictures of what I'm talking about, taken from my case:http://tinypic.com/r/140kke0/8http://tinypic.com/r/f2sayh/8http://tinypic.com/r/2ptp4c9/8