I've had same problem with my last 2 tops.
Had 2 gazebos pretty close to yours.
My wife pokes it with broom after a rain.
Let me know what works for you Those struts are weak to lateral
forces and I never wanted to attach any weight to them. They'll sag,
and you'll just collect water again.
The first gazebo we had there was never a canvas sagging problem, but
one year a heavy snow collapsed it, mangling the upper struts and
centerpiece hub. Now we take the top down in the late fall.
I just looked at mine. A couple of the middle canvas strut pockets
aren't even on the struts. Not sure is the canvas was cut too big, or
the struts made too short. Probably badly cut canvas.
I always wished they put length adjusters on the end of the struts so
you can pull the canvas taut with them. You have to stretch the
canvas pockets over the end of the struts, and I've had a number of
pockets tear.
This is cheap Chinese crap, so I don't worry much about replacing it.
We've enjoyed the gazebos, but don't expect many years from them.
Here's the solutions I think would work, if you're up to doing them.
1. Fabricate length adjusters for strut ends. You can draw the canvas
taut with them. Or shorten the struts with them to accommodate a
canvas that is cut too tight. I've bought 2 aftermarket canvasses for
10x10 gazebos. One was tight, leading to torn pocket areas, one was
loose. leading to sagging and water pockets.
The 2 obvious methods for adjusters are screw ends or clamping.
Never thought farther than that, except that I could do it with a bit
of thought and work.
2. This is easier, and if your canvas pockets are all on the struts,
might do it. No big loss if doesn't work.
If all my pockets were on the struts, I'd try it myself.
Tie a non-stretching string/cord/wire around the hub, then run it
under the sag, and attach it to the top horizontal rail.
Do it for every sag that collects water.
3. Sew or clamp a strap to the canvas edge where it most easily
eliminates the sag by pulling. You can tie it off to a lower
horizontal rail (ugly) or attach a small flower pot or decorative
object to the end of the strap.
I've noticed on mine that very little pulling force on the edge will
eliminate the sag. I might do this on mine. But poking the water out
with a broom handle is working so far. I'm just not too big on
worrying about it. Water sags drip through, but we don't sit in the
gazebo when it's raining. If you let the water sit long, it'll stain
the canvas from collecting dirt, but you get plenty of bird shit on
it anyway. Just not something I'm going to worry about.
Funny thing about those gazebos is I paid 80 bucks each for 2
replacement tops over the years. Both were a bit loose.
But I only paid about $150 each for the entire gazebo with canvas.
Both were tight, and the pockets tore a bit right off.
You have to shop around, or you can pay twice as much for the same
thing.
Let me know what works for you.