I've been using the mobile device made by Heartmath lately — https://store.heartmath.com/tech/. It looks at HRV (heart rate variability) and gives you a realtime plot, thereby offering visual feedback about how my breathing is affecting how steady the time between heartbeats is. Hearthmath calls low HRV "coherence" and getting into a lower HRV state (i.e. via regular, smooth breathing; maybe 1 in/out breath each ~10 seconds) is strongly associated with enhanced levels of calm and clarity — and I would strongly suspect (though can't know for sure) that similar effects would be seen as in the paper w.r.t. the immune system. I've found it exceptionally helpful.

I am on and off a long-term meditator (for about a year I was meditating as much as 2 hours a day). Some people don't achieve deeper states of calm and contentedness, in spite of meditating often (I've encountered this among serious practitioners of zen, for example; this happens to me sometimes, too) and I strongly suspect that lowering HRV would help.