It is similar to what they do out in Arizona and California except I had to use 1/4" hardware cloth instead of 1/2" as copperhead juveniles tend to be smaller than rattlesnakes.
I basically trenched down about 4" along the base of my fence and placed 24" hardware cloth along it from the trench up attached to the fence. That blocks them from just going through the fence. That leaves the gates. My gates are just goat panel steel tube farm gates set in the fence posts so not many good options to seal off those gaps.
Without getting very detailed, I got some UV resistant shade cloth and a length of 1/2" outdoors grade plywood 2' x gap length and some galvanized metal strap, and some 12x12x2 brick walk squares. I attached about 12" wide flap of the screen cloth to the sides and bottom of the plywood, then cleared out a flat area under the gate bottom and laid the plywood along the fence and set the bricks over the flap to secure the bottom edge like a big hinge, and attached the side flaps to the posts so they form a 45 angle side barrier on each end. The plywood can lay flat so the gate opens and closes over it, but I can flip it up against the bottom of the gate to create a barrier across the gate gap. I tried various ways of securing it up but the easiest was just to use a t-post to prop it up against the gate and then move the post and let the barrier fall flat on the ground. A quick flick of the toe and move the t post over to prop it up against the fence and it stays up. It's held up well enough for a few years now. I'm sure it could be done more neatly.
I have considered refining and patenting the gate barrier concept specifically for snake/vermin exclusion, though likely many such ideas already exist. There are definitely better materials and design details to use. I also think manufacturing rolls of hardware cloth with the bottom 6-8 dipped in a plastic coating would be good as sprinklers and dew and wet ground cause it to rust out and need repair faster than I would like even though it is galvanized.
It isn't 100% proof but in all the years I have had it up I haven't found a copperhead inside the yard. Other snakes that have more of a tendency to climb do get in now and then but copperheads seem to stay low and go along it and not try to climb up anywhere.