How about making something like a dresden plate for under the tree.
Use grocery bags to make a pattern of half a circle. Fold this half circle into three or four sections. This will give you either 6 or 8 "Desden Plate" pieces when you open it up. Cut out a small circle for the tree.
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Cut out the pattern. You can make a crazy quilt pattern out of any Christmas fabric that you have, if you don't have enough of any one fabric, and you can use solids in-between.
Line it or sandwich it with batting, turn it inside out and stitch in the ditch between the sections.
Just an idea........