I've been thinking about blocking my grill(s) for some time now. I've skipped the coroplast step because I wanted a very clean, factory looking look.
Version 0: (Kinda-prototype / proof of concept)
This is the first one I made. It worked great as far as blocking the grill, but the chrome nut needed to play chameleon or else the 'block would go to the scrap bin.
Version 2: Update & mod to V1 to make it stealthier
Added tabs inside the grill in a way the nuts would be inside the grill. Made bigger hole in the grill to make removal easier. This version look good enough for me, but I'll probably end-up doing one more update, because even tho it's not eye catching as the first one, I think there would be a way to do it even better. Anyway, this one is OK looking, and I am pretty sure it will help aero-wise.
Note: I didn't block the 2 smaller grills on the outside of the bumper. This is because the right one is 95% already blocked from the factory, and the left one feed my intercooler. I'll probably end-up blocking them anyway for the winter, when I find a way to make them the way I'd like them.
Edit:
Almost forgot. Cost goes as fallow:
4x Toggle Bolts (5.99$ for 12) 1.99$
1x (24''x36'') Bronze Plexi (20.99, used 1/4) 5.27$
1x All-Plastic Super Glue (6.99, still full) 1$
=========================== 8.26$ Cad