We're shooting for predator proof, so we actually dug 8 inches of dirt out of the run and laid down hardware cloth under the entire run. I used baling wire to 'sew' the overlaps together. Where the wire met the sides, we attached 2 x 6 treated lumber, stapled the wire (with overlap from the side walls) and then covered the staples with 1 x 4's nailed in. Making any sense?
Then we replaced the dirt & added chickens. Nothing will be getting in from below. It was probably more work to start with, but we're trying to safeguard the chickens AND be able to leave for a few days without worry.
Good luck!