I'm in the middle of my component install and realize the the following:
1) The Polk adapter rings are thin, therefore your mid-bass will be recessed in the door panel approx 3/4" more than the factory speaker. This may create a depth issue in relation to your mid-bass magnet and the window. I bought some thick plastic spacers on Fleabay for $15 and they seem to do the trick. I would avoid the MDF for worry of moisture issues here in Florida(at least for me).
6½" Speaker Mount Spacer Plates Adapters Brackets Ring Fits Nissan Inifinti | eBay
2) Tweeter mounting- I don't know if the Polk's come with the adapter that recesses into the stock tweeter opening, but Infinity's do. This eliminates the need to make a adapter from scratch. Pretty sweet if I must say.
I've had Polk, MB Quart, Kenwood, Pioneer, and Infinity components over the years. I have to say that I am most happy with Infinity, namely the 6030cs components. Great bang for the buck. Amazon has them for as little as $104/
pr.
Good luck with the install!