No other waterproof/water-resistant shelter can be more simple than the bivvy sack. If you can handle being in a confined space it’s a good substitute for a tent. They’re lightweight and extremely easy to set up. You can even pitch it just about anywhere you want. With a bivi, you can arrive at camp, crawl in, and go to sleep.
Stormproof Bivvy
If water can’t get inside a bivvy sack, it is considered stormproof. You can use a stormproof bivvy sack alone even in bad weather. The fabrics, seams and opening are channels where water can seep into the bivvy sack.