If you go synthetic recommendation was given on Arrowhead check out his site. For example the Potomac 1lb 12 oz 30* full length. Your not backpacking much and summer rarely gets to 90* and that's July. My suggestion is go to REI downtown store in Seattle grab a bag goose or synthetic walk over to the hammocks then you'll have a better idea; weight, size etc. Look for a Big Agnes bag flip it over.
Right now I think they are in their winter moving gear toward summer. Only you know the many factors; type of sleeper, gear, etc. what is known is your wet environment logic says synthetic. Me I'm goose also kayaking using dry bags. Storage use the large bag included and hang but not compressed. Finally I started with synthetic no harm having both.
Enjoy
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