With M50 I made bivy - very waterproof, not supposed to be real breathable but I haven't noticed a problem, very flimsy but I was able to sew it okay and it hasn't ripped or anything. Very downproof.
I also made rain jacket. I'm waiting for opportunity to test it. Tomorrow's supposed to be good opportunity.
I keep using my pyramid - works good. Quite a few people have made something similar. I'de probably do the same today.
One thing is, for solo, I've been experimenting with "half pyramid". You leave one half of pyramid as is. For the other half, turn the rectangular area into a triangle (or beak, or vestibule). Saves a little weight. Put a zipper on ridge of the triangle.
You can buy 2nds silnylon here from many sources. Maybe that's what you have. It's a little less waterproof. When I've made a pack, and had something inside like puffy, water wicks through a little. Or when I sleep on it, water wicks through a little. If you coat it with mineral spirits:silicone it makes it much more waterproof. For a tent, it's fine. Pyramid never leaks. Except I get a little "misting". When it rains hard and I'm sleeping underneath, I feel a very slight mist. But it's not enough to get anything wet. Really just aesthetic.