Maximizing retirement plan contrib after closing practice

[buckstar]

I closed a medical practice early this year and let all of my employees go. I had a 401k plan that has been terminated that had a safe harbor (4%) match for my employees. The practice is no longer open, but technically the business is still active, I'm still receiving insurance payments, etc... Can I open a SEP before the end of the year and contribute more to that account? What would be the maximum I could contribute if my employee contributions have been 18k and employer contrib 9k?