Could anyone give me advice on how to get my box hedge to square off

You can only achieve that properly once it's formed a proper hedge, in that the plants have grown sufficiently to close the gaps. In the meantime, use shears to clip the sides off in a straight line, and to flatten the top.

As Bamboo says, go ahead and start clipping the sides and top now--that will speed the growth between the bushes, where you aren't clipping. Also clip them so they are narrower at the top than at the bottom. Allowing them to go the other way results in bare stems at the bottom. If they still need to put on some size, just clip them enough to nip the growing tips off on the eventual sides and top, to thicken the growth.