L-brackets usually suffice, or you can remove the drivers and bolt the box directly to the vehicle. If you choose the latter approach, be extremely careful when drilling holes. It's amazing how much damage can be done to a vehicle if you don't look where you're drilling. Now you can give yourself a pat on the back for enhancing the performance of your auto sound system. I've built many enclosures over the years, and with each one I've picked up a new trick or two.
Patience is key, but it is even more important to talk to manufacturers and experienced installers.
Following their directions makes for smooth sailing on the road to good sound.