It might be easier to just add another fan to your case to an empty opening if you have one. They also sell Y cable adapters that will split your sys fan outlet to 2 outlets. They even sell adapters that turn one connection into 4 outlets for sys fans. That would cost less than 10 dollars.
I'm not sure you are going to get much air flow out of a 3.5 opening. If you case is really sensitive to heat, then I would splurge and get an aluminum case.
I've also seen some plain metal computer cases for less than $40 that are better than the ones they were selling 3 or 4 years ago. They are thinner gauge metal (lighter) and have more fan slots.
I've also seen some local dollar stores with office desk accessories made out of metal mesh. The big problem there is either taping or protecting sharp edges from cutting cable ribbons, fingers, etc. Using auto cable sleeves secured by plastic ties around sharp objects is a sure bet.
I've hot glued mesh to cases, but after all the time and effort really didn't see that much of a difference than I did switchng to more expensive aluminum cases (up to 10C). You get what you pay for. I feel your pain though. I have my eye on a server rack box that costs about $250.
Maybe when gas goes back to less than $2.5 a gallon.