There are infinite ways to do this; there isn't really a "right" or "more appropriate" solution unless you can expound on what exactly it is you're using the fan array for, if you want to be able to move it around, etc.
If you have access to a 3-D printer I would suggest making tiny little square plates with 4 bolt-sized holes, sized so that when centered at the mutual corners of 4 fans (the center of a 2x2), the holes line up with the fan's mounting holes, and use 6 of them (3 to each side of the 4x2 array) with 12 bolts to secure them, which should secure them together, but obviously won't provide extensive rigidity.
If you have 80/20 T-slot lying around, sure, you could rig something with that. If memory serves correctly, the mounting holes in 120mm fans tend to be just big enough for 1/4" bolts, which means they'll fit into 80/20 T-nuts just fine. You'll want to come up with some kind of frame to mount the fans on, but keep in mind that 80/20 is a fairly bulky and heavy material, so that the more of it, the more cumbersome the design will be.