I was out playing this morning at Combat Action Games. Nice warm day easy 20 degrees.
I was in my usual attire. Base layer, black ubacs, boots, trousers and warrior zip up load bearing vest with all the bells and whistles, topped off with gloves, cap and eye pro.
The single most important item on that kit on a hot day. Hydration bladder with 2 litres of chilled water in it. Heat isn't the issue. Dehydration is the issue.