If you don't mind playing with fiberglass (it's easy), then it shouldn't be hard at all to make the stock grille work. Some time ago I did a write up on a DIY brake duct system ($90 all in). I can find the link to that later if you're interested. Here's the super short version.
1. Modify the OEM grille (easy, be patient, go slow):
If you're doing it right, this is what you're looking for:
2. You should be left with this:
3. Moving on to the backside, you get to play with flower foam! (This stuff shapes super easy, dusty, but you can practically just look at it and it will conform to the shape of your willing.)
Note that this is a very rough form still. You should smooth it out more before glassing it up. If you've never tried this before, relax, as it is hard to "F" this up. The best way I've found is to coat the foam buck in resin, sand it smooth, and then wax it 2-3 times (acts as a release agent). Then do your wet lay up, and after an hour or so, it should be relatively easy to pop the buck out of the duct. If you don't do this, you're still good. You'll just have to cut the foam out of the dried duct. It's messy, but the foam offers little resistance; it stands no chance, it'll just be messier. As I said, you really can't mess this up.
4. Here's the final version from the back (Note, it would've been easier to just glass the whole thing using the grille itself as the base, laying the fabric over both foam and grille. Of course, you'd want to mask off the grille so that it wasn't a permanent attachment and you can pull the duct off of it.)
And from the front:
Easy as pie. Just one $15 box of standard automotive fiberglass kit should do it (I used epoxy resin, but it's not necessary for this application).
Best,
-j
