I have instructions on my website for making your own synthetic underquilt. Check them out in the Articles section.
I use 1.1 oz DWR breathable nylon for the shell. It's not waterproof, but it breathes well and will repel light spray or misting.
Synthetic is better than down when wet, but getting it wet is still not something you want. Most people find that they can use either and keep them dry by setting up smart and using a good tarp. Keep in mind that water running down your hammock and/or underquilt lines can be as much of a problem as anything, so you'll need drip strings or something similar to divert it.
"Keeping You Warm Underneath"
On the web at:
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