+1 on the blanket. Used one myself for years.
If you want a lot more effective Owens Corning acoustic panel is the standard material inside a lot of very expensive acoustic panels. Two 1" thick panels 2x3 ft should do it. Wrap each one with fabric, done. So light and small, easy to move, extremely effective. Being 2 panels easier to move and you can experiment with things like using them angled or flat or whatever.
If you want a lot more effective Owens Corning acoustic panel is the standard material inside a lot of very expensive acoustic panels. Two 1" thick panels 2x3 ft should do it. Wrap each one with fabric, done. So light and small, easy to move, extremely effective. Being 2 panels easier to move and you can experiment with things like using them angled or flat or whatever.