At the risk of repeating myself endlessly : ) fleece doesn't provide much warmth for the weight
I have synthetic booties for just inside sleeping bag. My current ones were sleeves I removed from a jacket I made. 2.5 oz Apex.
You don't have to do much more than make sheet of fabric, insulation, fabric, fold in half and sew one row of stitches, sew row of stitches to enclose end, kind of angle it to make foot shape. They cover calf, just stay on without elastic.
Or, you can hide raw edge. Make a French seam. Cut off raw edge close to seam, turn inside out, and do another row of stitches enclosing raw edge. http://thru-hiker.com/projects/french_seams.php