So you want like a soft gutter fixed to the underside of the top and the glass so the rain runs out the ends? If so i think the water would then pour out the ends causing more problems.
I think a rear awning is your best option given you have a rack plus they are pretty cheap, I have the short rear one on as a side awning and they set up pack down pretty quick, not great in strong wind but I doubt you would be tailgate cooking in strong wind