Fill gap on slope with horizontal fence
Just put another row(s) of boards in. You can scribe &amp; rip the boards to follow the ground, keeping them off the ground by 1/4". Or, you can dig out the ground for full height boards, take the dirt from the left &amp; put it on the right for a foot on both sides of the fence.

Assuming the boards can't be bent enough to slip them in the channels. Cut the boards in half or wherever you find most appealing &amp; least obvious. Half-lapping the cut zone would be best, but you can also just screw the boards back together with a board scrap.

Really Nice job by the way. I'd prefer to keep that color contrast as long as possible. I'm on a One Time Wood Protector kick, if both sides or just your side gets a few hours of Sun a day it's the only way to go. If the fence is wet in the picture One Time's Natural will keep that. Not a promo, I just find that no-one knows about the stuff &amp; they just keep ruining their lives with poly's.