You're not going to like my answer, but I'll give it anyways.
Use an attorney. I know it sucks paying someone $250 / hour to write a short paper for you, but honestly, you can pay him $250 an hour to write a good contract (Maybe $500 total), or you can pay him $250 / hour + expenses ($10,000-$20,000 minimum) to go to court when your bad contract throws you into the fire.
Good contracts make good businesses. More-so when you're looking at investors. You have a fiduciary duty to your company to protect it, and yourself from controversy. And having good contracts, and seeking legal advice when it's needed is part of that duty. Your shareholders can actually hold you criminally, and civilly liable for failure to satisfy fiduciary duties.
Keep everyone happy, and get a lawyer :)
There's a Y Combinator startup that built a generator for convertible notes, Clerky. I am working on a public repository for this kind of templates, too.
Even lawyers start from templates they found on the web. They should be free and accessible to everybody, and not from sketchy websites like blitzdoc or other clones.
Update - some convertible note samples here: https://www.docracy.com/doc/search?query=convertible+notes&s-submit=Submit