How can I keep the top of my dog ear fence level when the ground slopes?
You will likely have to make them out of 1"x8" x 8'.  If 5 1/2" wide would work, 1x6 would need no ripping to width.

Be sure to catch table saw/miter saw dust with a shop vac and a dust mask!  PT sawdust is toxic. 

Be sure to use a compatible preservative on the top dog-ear cuts (where the untreated wood gets exposed). End grain is very susceptible to rot.