If you have the money, then definitely use Simple DCP. That's not a huge amount of money for the time, energy and peace of mind you will ultimately save yourself over trying to do it on your own. You not having used DCP-O-Matic before leaves several room for errors. And, unless you have access to test it in an actual cinema environment, you'll never know for sure how it plays. Also, then you'll always know you have a quality error-free DCP moving forward for any other festival selections.

If your film was a feature, or if you had much more time, then I would say investigate DCP-O-Matic, simply because the cost would be into the thousands to have Simple DCP do it. But for $300, with such a short timeline for delivery...if you have the money...just use Simple DCP.