In reply to Big Jim The Climber:
Although I am sure the manufacturer might help you (Force Ten did), but I tend to attach the tent to the outside of the tent. Hence, I needed something more durable and definitely waterproof.
Once it started to rain at the evening, it rained all night and when I woke up, I was in the cloud until midnight. I think it was rainy and windy for 14 hours or so. I decided to buy a drybag instead. I think it's 14l or something, but I just use two straps to attach the tent, tighten the straps and then roll the dry bag.
These bags look really good:
https://www.alpkit.com/products/airlok-xtra-dual