FrogReaver
The most respectful and polite poster ever
So if I can't come up with a wording, my position is wrong. When I do come up with a wording, I'm still wrong. Why am I still debating this with you exactly?
You asked for a version of the rule that lets you get an extra shove on your turn. Why does this have to be a bonus action exactly? There's a precedent in the rules for giving you extra attacks as part of your Attack action, for example the Extra Attack feature. Dread Ambusher also works in this way. If the intent was that you've become so good with your shield that you just get a free extra shove on your turn with no timing restrictions, why not word it that you get an additional attack as part of the Attack action that must be used on a shove? That way we wouldn't have to spend another thousand posts arguing about the duration of actions or how to correctly declare your action ahead of time so the reaction rules don't put you in an inconsistent state and all that.
The goal remember is for you to write a rule that would make shield master function like it does under my interpretation. My interpretation requires a bonus action to be used. Not including a bonus action fundamentally changes the rule.
That said, if I were you I would answer something like: you may bonus action shove on your turn provided that you haven't already taken an action other than the attack action. If you bonus action shove and haven't yet taken an action then the only action you may take is the attack action.