Chris, this is the only other photo, which is basically the same only brighter. It was hard to get a good shot under there.
The rubber I used was from an old inner tube. I cut a square about 4" x 4", with a slit to the center from one side. Then cut a 1/2" diameter circle at the end of the notch in the center of the patch. I used self-tapping sheet metal screws in pilot holes to secure it, along with the Gorilla tape. The tape originally just held the patch in place while I drilled and ran the screws, but I left it on and added some to seal the edges a bit. Word of warning: have several metal drill bits on hand. Drilling into the frame or sheet metal around the holes is hard with small diameter bits and they tend to break. And drill the pilot holes slightly larger than you might normally in softer metal or the screws break, too.
On my rig, there are only 3 exposed hydraulic rams, so it wasn't an all day job.
I think the person that posted this idea originally may have purchased a sheet of rubber material to do theirs.
View attachment 12060
While we're on the subject of plugging frame holes, I taped a piece of plastic screen material (pet proof type) over the holes and then used the expanding spray foam over that. Seems to work and has held for two years. Didn't like the idea of trying to seal the holes by just dumping a ton of foam into them.