You didn't mention the firearm you would be carrying, but under a tux I would go with a smaller frame model. Of course the classic formal wear gun would be the Walther PPK .380.
I own a Miami Classic II for a Sig P229 and while it is a fine rig I wouldn't consider it for under a tux due to the fitted nature of formal wear.
My first choice for a shoulder rig would be K.L. Knull Holsters. His rigs aren't as well known as some of the larger outfits, but are first class. His shoulder rig was originally designed to carry an Automatic Ingram's MAC 10, for government agents and required custom fitting for the wearer. His shoulder rigs are extremely comfortable which I can't say is common for that type of rig.
My second choice would be Andrew's Leather, but anything would be a distant second to Null.
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