If you fire up a VPS somewhere and install CapRover (pretty easy for even someone virtually nontechnical), I recently contributed a config for Mattermost to the CapRover "one click apps" repository, which does just what it says on the tin: a single click to deploy your own Mattermost instance. (My config even turns off the on-by-default Mattermost server spyware, so you're private-by-default.)

https://github.com/caprover/one-click-apps/blob/master/public/v2/apps/mattermost.json

Drop me a line if you need help setting it up.