Hello Andy,
it seems that you’ve gone from needing a way to tie down a sheet of plywood - 4ft x 8ft, to requesting help for a door that at most likely is 3ft x 80 inches. Different enough to require other procedures for securing.
With the sheet plywood, the anchor points would most likely be under the sheet at least a few inches, but in your diagram the item (door) is much narrower than the anchor points.
I took a stab at a diagram for you to consider. I’m sure there are many others. I hope it gives you something to work from. Since you have indicated this is to be a door and that you’ll travel slow and easy, I feel this will be secure up and down hills and curves barring any undo outside influences.
You potentially could wrap a sheet of plywood similarly, but that is entirely up to you.
All runs of the rope should be cinched as tight as possible throughout the tying and the frapping in the center crossing should tighten it further.
