For a while now, the fans in my desktop machine have been mounted in the middle of the case. This makes them quieter than normally mounted intake fans because they are further away from the front of the case, but they still draw most of the air through the front grille, making it easier to filter dust out than if they were mounted as exhaust fans. Recently I replaced the 80mm fans I was using with larger 92mm fans, and therefore had to construct a new bracket to mount them. I thought it might be useful to take some pictures of this and post them here.
The bracket is made from carbon fibre because I happened to have some spare pieces of it lying around, but any material would work. In the first photo you can see the two halves of the bracket at the front. Behind is one of the original pieces of carbon fibre from which they were constructed. Two of those were required for each half of the bracket, connected together by gluing an offcut across the middle. I used a cutting disc to cut out slots to attach the fan to, some curved sections where the fan blades are, and some slots to allow it to fit around the sticky-out-bits in the drive bays. Then I removed the sharp edges with a small round file (Carbon fibre makes nasty splinters, so it's important to keep the edges clean).
The second photo shows how the two halves hold the three fans together. They are attached to the bracket with the rubber things that came with the fans.
The third photo is of the entire assembly mounted in my PC. It fits precisely into the 3.5" drive bays, held in place by some thumb screws at the sides.