I beg to differ. Check into what the ultralight hikers are doing, a lot of them seems to arrive at the conclusion that the ligthest equipment is DIY or heavily modified.

The way I see it, if you have these 3 requirements
- A need/want slightly off the main stream market (and a 2-persons bivy bag appears to be)
- Good quality
- As lightweight and small as possible

Then your options are either ridiculous expensive or DIY.

DIY does not nescessarily means you'll be doing the stitching. I'm sure you can pay a seamstress to stitch together an old rainfly to suit your shape. Making a cheap model for testing might be a good idea anyway as it is much easier to see what works and what doesn't.