There is the SOL Escape Bivy.

You could make your own UL bivy pretty easy using a variety of off the shelf or reclaimed materials. I made a UL tent using SOL Emergency Blankets, clear Duct Tape, and a bit of sewing. I'd think it'd be pretty easy to make a bivy too.

The Eureka Solataire is light, and you can get those for under $100. I had one, but found I'm not really a big fan of bivys after using it.
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