that is a big hole where cold night air will sink under your house and be trapped.
Can’t it get back out the same way it came in? 
I know, I know. I’ll have to tack it. Maybe do something next year that looks a little nicer.
the plastic will rip off the inside application due to wind, unless put up with strips, before the plastic applied to the outside.
for long term year to year use; make rectangle frames of 1x1 which you attach plastic to. fit to the structure from the outside and place in the same location each year. heavy sun exposure will deteriorate the plastic in maybe one year.