I have that bottom piece broken in the same fashion as well as the rear bracket break completely off, leaving a giant hole. Personally I left it - taped up the hole and put some foam on the bottom to keep it quiet - because it's now sooo much quicker to remove from the engine bay when need be (and new air boxes are like $150 from a dealer). The brackets are just there to keep it from moving around and rattling, nothing real crucial. All V6 3rd gens used the same air box so you shouldn't have much trouble finding one from a junk yard if you're set on replacing it
It'll be fine as is but if you want it a little more secured, you could drill a hole in that upper bracket and run a zip tie through there and the corresponding bolt hole. Could also clean up the break some and epoxy on some plastic or metal to make an extension; when you think about it the bracket is just an arm with a hole in the end, and a replacement wouldn't have to be any more complex than that. It just comes down to how clean you want it to be