This is military parlance, or English that is specific to the military.
"Solid box" would likely refer to obtaining a missile lock on a target, and the pilot or gunner is informing his commander that he is able to fire. While missile is seeking a target, the image its camera is transmitting to the gunner's screen has a flashing rectangle (box) super-imposed around the intended target. When the box stops flashing and becomes "solid", the missile is locked. The gunner might then inform command that he has a missile lock by using the term "solid box". Be aware that this is just an educated guess, and I'm not 100% certain on it, as I've never heard the expression used before.
"Solid copy", on the other hand, I have heard before. It means that the person has clearly understood an incoming message over their radio. It is used when there is absolutely no doubt that the message received is the same as the message sent, and there is no radio chatter that might cause it to be misheard.