What to do with personal withholdings funds?

[Andrew Kniffin]

This year, for the first time, my wife is self-employed.  

This means that, rather than have an employer take out x% every two weeks, she gets a full payment for her work and then we must pay $XX,XXX to the Feds 4x per year.  

What is the smartest thing to do with these funds?  They need to be liquid since I'll need to drawdown on them every ~90 days.  But putting it into a savings account seems like a waste.  

There's got to be a better way. Can I provide short-term funding to a HML? Or....? Interested in anyone's wisdom and/or input on the topic.