I have several inexpensive meant-for-workers short sleeved orange mesh zip front shirts that I wear over whatever else I am wearing. I believe I bought them at Wal-Mart and I think they were branded Jesse James - but I can't find them now though either web-site. Medium fits me well with nothing bulky on, and XL fits over anything else I wear (down to -30 celcius).

I like the shirt style rather than the vest since my arms have a wide band of reflective material around the arm to increase lateral visability.

But - I don't often ride on drops - neither my commuter, my mtb, nor my utility bike have drop bars - so I am more upright than some of you.

And - too hot is not often a problem where I live.