I cut the bottom out of a vinyl gutter and used that to isolate the MCA treated 4x4s from the frame. Here, I heated the vinyl and used a routed 4x4 to mold the vinyl around the rivets and bolt the protrude on the top of the frame.
Bumps from the rivets
I used a routed piece of 4x4 to form the bumps in the vinyl
This is the attachment. 1/2" stainless steel bolt