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I just double-checked and it seems my estimate was a bit off. While mine isn't less than 0.25" doubled up, it is closer to 0.25" than 0.5" in a single layer. It's about 0.4" or 0.5" doubled up. ...

They're pretty flat, normally. Mine have had ~0.25" of loft, sewn-through. Remember that these were intended originally for temps in the fifties or so.

As far as cleaning, there should be a care...

That sounds about right. If memory serves, a liner is 7' x 5', and you fold it over to make a PLUQ that's roughly 3.5' x 5'. The 5' is in line with the ridgeline. Leastwise, that's how I did it.

It might be worth putting a few hand-sewn basting stitches (long, wide stitches that aren't meant to be long-term) along the sides to make that easier. It might take a little more time, but...
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Did that with mine, except that I just ran it through the edging on the liner (same as he suggests with his no-sew version). It works okay; it's kept me warm as low as 25 F with no supplemental...