Honestly, I wouldn't bother with the 6x9s. Properly tuned front speakers + a sub will be plenty. What do you have for amp and head unit? Let me know what 12" you have and I can help you design a box.
I haven't read any good reviews on saddle blanket seat covers. It sounds like they don't make them like they used to. I'm just going to do a new cover from Acme via rock auto. If you need new seat bottom foam you can get thru ebay:
https://www.ebay.com/itm/12548992-MF...0/162357301815