growing sweeter grapes under a plexiglass?

Hello.

I am building this pergola and will plant several grape vines around it so that they could climb on top. These are table grape varieties, like Concord, Thompson, etc.
I would like to make a guess that under a sheet of plexiglass the grapes will be sweeter because of warmer air and the glass will prevent birds from eating the grapes.
Am I right or wrong?

Also, my concern that the gap between the plexiglass and the rafters is very narrow (2") for the vines to get under the plexiglass on their own and I will have to train and guide them manually first.
Am I right or wrong?

Thank you for your comments.

Neat idea. Your gap seems fine,2" is huge for a grape vine.
If you only cover it with Plexiglas on top, the birds will still get to the fruit, if you surround it, you have built a greenhouse.
Either way humidity could be a problem,leading to disease.
The main benefit would be a delightful place to sit and watch your garden,during a rainstorm.