DIY quilt fabric

My personal preference is RTBR .66 taffeta.  It has a good feel to it.

Well, a lot of the clammy feeling is due to lack of breath-ability. The Momentum fabric has a good DWR coating on it. In most cases, the DWR will reduce breath-ability.  Try an untreated, but calendered, fabric on the inside. This will allow it to breath a bit better (30-100%) and allowing the down fill to soak up any humidity.  This lets it compress a little (reducing warmth, yes,) but also reducing the next to skin temperature, thus reducing your tendency to sweat. You will reduce claminess a lot, making you more comfortable, overall. In some cases, a too warm bag can do this. For example a 20F quilt after an hour, can feel cooler and clammy on a 40F night. Anyway, these are my top three choices:
https://ripstopbytheroll.com/products/0-74-oz-mtn-7d-ripstop-nylon?variant=3259760476186
https://ripstopbytheroll.com/collections/ultralight-nylon-fabric/products/0-66-oz-membrane-10-taffeta-nylon-1?variant=11055919489
https://ripstopbytheroll.com/collections/ultralight-nylon-fabric/products/0-56-oz-membrane-7-ripstop-nylon

I think these are the best of the ultra light fabrics for quits.

It will be a light apex quilt for summer so not too worried about max DWR.

What is RTBR?

Never mind. I got it. Rip stop by the roll.

M50 is good for outside.  It has very good DWR.  I was just cowboy camping, there was a light drizzle, I just ignored it, normally I would quickly get up (in the middle of the night) and set up tent, M50 repelled the drizzle just fine.  Also, it’s good for condensation.