I don't know how much longer or wider etc your lip kit makes the car, but I have a Covercraft Cover and it uses elastic to pull in the bottom at front and back. It's my guess there's enough fabric in the cover to easily pass over a reasonable kit, but without actually trying it, I certainly don't know.
I went with AutoAnything.com. Covercraft has several versions and I went with "Noah" as my best guess fit for my needs (I also specified a rear spoiler). It cost me $213.26 shipped last May.