I received this Orvis Poplin Vest as a gift and man do I like it!
It has a net backing that lets air in from the slits in the back and in the 100+ temps, it's not even noticeably warmer than just a light shirt!
It covers my IWB holster very well and might even cover an OWB holster.
Only one drawback, it's pricey! But man it's nice!