My BF started running this year (yes, the pandemic does not have only negative effects) and despite myself buying him a few running clothes pieces, he definitely prefers his RTW merino t-shirt. This piece was just in such a regular rotation, that he almost wanted to wear it two times. So even though merino does not smell bad even if you sweat in it, I do not think it is always a great idea.
I just remembered I still had about a meter of merino sports jersey from The Fabric Store and decided one week-end to cut into it. It was originally "reserved" for a cardigan for me, but I am still undecided which kind of cardigan I would like to have, so that is not happening any time soon. I believe a sports t-shirt will have a much more utility now.
This is just a basic, so in terms of sewing, there is not much to say apart from the fact that I copied his RTW t-shirt and sewed it with a flatlock seam. I wanted to add a little something, so used an old scrap of cotton jersey around the neck and a little "etiquette" on the left side for days when he does not now where the front and the back is.
The t-shirt got instantly worn and is in regular rotation since - my BF currently runs 3 times a week, so he is at least covered for two of them with merino. On the other side, the weather is getting warmer, so I believe that he will start wearing regular sports shirts soon.