The last baptism I went to, I wore a tweed suit. Most people there just wore blazers (navy or tweed) + chinos or flannel trousers (very SF of me to notice right?). I don't think there is any etiquette for baptisms today really.
@mcobinad, check out @Treviorum just a few posts up. His DB suit is rather slim fitting, yet there is nothing straining or constricting about the look. I think that kind of fit would be both more flattering and comfortable for you.
Oh, and since you seem like a guy who lifts, you'd probably need to go MTM or bespoke.