If you want to try to increase stability. Remove the legs from the hip, and put some clear nail polish on the balls. I have two coats on mine, and it stands pretty well. With the backpack, just.... get used to some floppyness.
PErsonally, my 21FP is staying in fighter without the packs. The sleeker body shape looks quite nice. I have my older non-FP behind it, in battriod mode.