Years ago I "converted" my Tricross triple to a 1x9 by just putting flat bars on it, ditching the front derailleur, and keeping it in the middle ring. It was perfect for commuting on flat roads and gravel trails. I ended up selling the bike but if I had kept it, I could have just gotten a new crankset with a narrow wide chainring (or even just keep the existing cranks and remove the small and big rings). Chainring and cassette choice will be based on your riding needs; the 39t middle ring with the original 12-25 cassette worked fine for me.