Finally got around to doing more on my car. I made a new lower grille block with coroplast, put a belly pan on the back driver's side corner, and put the stock air tube back on (to see how hot the intake air is with it and the grille block).
I originally had it only attached at the front right, back right, and the middle of the left side. This made it sag on the right side, partially because of the crease I had to make in the sheet for it to fit in my car. To fix it, I ran some zip ties through the frame rail holes and used that to pull up on it. Should I close the hole at the front inside the wheelwell (where I took the last picture), or will it make little difference? Now I'll have to see about a smaller muffler when the current one rusts off so I can do the same on the other side.