Detach the hands at the shoulder ball joint.
Transform the legs like this. The guns should go into the holes on the side of the arms, then into the holes on the legs.
Like so!
Okay, so he's still a little hollow, but at least it bends at the elbow.
An alternate mode. (Yes, you might've caught me forgetting the chest 'wings'.)
How it works from the other side.
Now if you refer to the game you'd realize I have the flamethrower with the boosters on the wrong side, but the underside of the boosters don't look great at all. You can swap Blast Off's guns and turn the flamethrower the other way for a third mod.
