The cover on my 5*10m poly-tunnel (approx) ripped one night in a strong
gale. (right across the circumference, and half way along the length)

Taking some thick twine, and punching holes in each side every 2cm or so,
and then lacing up the split edges has worked well.

Start out with a fairly loose gap, and then you can gradually bring the
edges together.

So far has lasted about 3 months, in fairly strong winds.

Tips.
Use a blunt pointed object to punch the holes - a sharp one will mean
it's easier to tear the string out.

Start with a long length of string - twice as long as you want to work
with, pull halfway through, then work to the ends - this will make
it easier to tighten up later.

I found it easier to lace it so that it on the outside, it looks
like dashes along the sides, with string crossing on the insides
directly across the cut.

I used baler twine, which is a 3mm or so plastic string, which can easily
be melted into a nice point.

Not advocating this, just thought that others might find it interesting.