The bottom line is what is the IRS going to want? You can put this on Line 21, but you will get a letter. You will have to explain your reasoning to them. They will want penalties and interest.
Was the one time work one day? Was it once a week over several weeks? Was it every day for a week? Was there a profit motive when you took the job?
Thirty years in the tax business and close to 20,000 tax returns has me advise you to put this on Schedule C.
