Polo shirts work good for some of you skinnier fellows, since reaching all the way across and lifting with the opposite hand is required for the draw. Either that or lifting with the same hand and then drawing.One way takes two hands, the other is slower.
I sympathize with the loose over shirt. If it's likely to blow in the wind, it's also likely to be clingly and hard to brush back to access the weapon.
If I wear a short sleeved shirt, it's a heavier one---like Cabela's Safari or something similar. I've found it not to be too hot in N. Nev. in the summer.
My favorite carry is the vest. Especially the one made by Bratwear. Just call in, they'll take your measurementsby phone and custom make it for you. It looks reasonably nice with black suede coller, has big pockets, including inside zippes on both sidesr for BUG, and it's compfy in the summer--at least for me. It's long enough to accomodate outside the belt carry.
Wasn't trying to change the subject away from polo shirts---they work well for some. But something else to consider.