My favorite shirts so far are TriBlends made by NextLevel. There are a couple of other companies that make them.
http://www.amazon.com/Next-Level-6010-Mens-TriBlend/dp/B0072S1W9Q
TriBlend=cotton, polyester, and rayon
They feel more like cotton, but still have the stretch and wicking of the polyester. No 'plasticky' feel and they don't feel like you're wearing a wet towel.
One problem though...they run small. I normally wear a large, and in this brand I have some XLs that look like (s)mediums on me. And it seems to differ by color. Go figure?
So my suggestion is, have your screenprinter get some of your color choices, in different sizes for you to try on and check the color. Most people that normally wear a LG....I'd get XXL. These shirts must be made for the bodybuilding crowd I suppose
Not any real good HiVis colors though. Their orange is more of a peach and that was one of the shirts that looked like a medium. I gave those to the girls
For working in the heat comfort, these can't be beat.
Side note: You could always make your screenprint the HiVis portion of your shirt