Found some pics.
With the fancy-pants moulded columns of my proscenium it isn't all that easy to make out what is actually going on with the play board

The play board does extend a few centimetres 'inside' the booth frame.
But my proscenium uprights also rebate 'back into' the booth, making it tricky to make out what is going on.
Because of the thickness of the column bases it 'looks' like the board doesn't stick out much at all from the booth. But I can assure you it does, and is about five and a half inches wide.

In the performance area the board narrows to about three inches .
During the performance , or with the curtains closed the audience is not at all aware of this.

The clearest photo of the board on its own is this one ( the play board is at the top of the photo)

Just another option for you to consider: you can make your play board exactly as per your diagram ...or you could jigsaw out some of its depth , allowing you to perform your puppets just a small amount 'outside' the proscenium , and nearer your audience.
If you use this method , I would recommend a gentle angled slope between the two depths as I have done for maximum strength.
A ninety degree cut would weaken things a bit I feel.
Good Luck with it. ( as always ..photos please !)
Richard