i would make tubes not squares, for the blanket with the foot zipped closed.
then i would make rip stop nylon tubes to fill with the poly beads and sew then closed.
slip your tubes in and zip closed. do not try to wash rice or beans tube!!!
i bought the poly beads from a doll making site years ago. they are also used for bean bags. you can remove the tubs for washing the blanket. use the golf club inserts for golf bags to fill the tubs. a twin size is for one person.