Here’s how we made one: https://www.frugalfamilytimes.com/2013/03/diy-make-huge-frame-for-way-less-money.html
Ikea makes some really cheap but nice-looking ones if you decide the cost of supplies and time is too much!
Here's some ideas from pinterest, they have printable ones too:
i love these rusti
c picture frames, look easy to make :) http://ashadeofteal.com/christmas-mantle-update-how-to-make-a-rustic-wood-frame/