My plan was to completely wrap half my run with plastic including the sand box under the coop, leaving the other 6 x 18 completely open.
I plan to leave openings for the chickens to pass between the covered portion and the uncovered.
The image below is an approximation of the wrapping I was planning.
Is there anything wrong with doing this? Is there anything in which I should be mindful?
So you are planning to run a piece of plastic across the middle of the run?
I think plastic will be easier to attach, and will hold up better, if it is attached where you already have fencing. In the middle of the run it might flap around (could scare or injure the chickens, and might rip the plastic loose). Or if you make it secure in the middle of the run, that probably means you put in a bunch of extra framing that you would otherwise not need.
Why would you want the plastic in the middle of the run?
If you are worried about wind coming from that side, you could put plastic on that side of the curent run, where the fencing is. And if you do not expect wind from that direction, skip that piece of plastic.