It depends on the type of mantel you want. You can go with something as simple as a wooden shelf using oak plywood or a thick wooden beam.
Here is a helpful video: https://www.youtube.com/watch?v=zffoGuVNOpM
Here are some ideas! https://www.hometalk.com/search/posts?filter=diy%20fireplace%20mantle
Here’s how we built a reclaimed wood looking one. https://www.frugalfamilytimes.com/2019/05/how-to-build-faux-fireplace-in-corner.html?m=1