Funny! I have the exact same Shad SH40 installed behind my Z650! Really handy.
It'll fit a full face helmet with enough room to squeeze a tightly folded jacket inside. Great for a quick stop at the groceries too.
I've ridden with it in some crosswind conditions, It does feel a little weird in high speed windy circumstances.
Love it to bits still! I had it on my old bike and kept it to install in the z650 when I bought it!
Best investment ever!