Firstly if your horse used to stand square normally & now he doesn't I'd want to check out any physical causes. It is normal for horses to stand not-square, but if it's a recent thing or a change, or substantial, it could well be a sign of a problem.
I don't think it's stupid to place his feet where you want them actually. Obviously it would be a bit silly doing this in a show
but it can be a step in training... pardon the pun! You've just got to work out how to transfer it to the horse doing it himself when you ask.
The way I taught my horses to stand square was to start with them already knowing how to 'stay' on cue, and to move a certain side back when I put pressure on that side of their chest or nose. More or less pressure means more or less movement. Then I 'refined' this to pointing at a side. When they're perfect, you just ask them to 'stay'(or whatever cue). I'm not in the show scene, so I don't know how most people go about it, but this worked for me fine.