Not strictly a bivy, but Hilleberg tents make a 1-person, 4-season tent, the Akto. A little heavier than a bivy bag, comes in at 2lbs 12oz. A good bit pricier too - $345.

However, everyone I've ever talked to have raved about the build quality, and how the comfort of being able to stretch out, and having good ventilation, was worth the extra dollars, and the extra pound or so of weight.

I think I know what I want for my birthday......

Irish
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I didn't spend nine years in Evil Graduate School to be called "Mr Irish", thank you!