The thing to visualize is that I am blocking out as much light in the room as possible so that I can control where and how much is let in. Often in the dining room, it is as simple as closing the vertical blinds and flipping up one or two of them to drape back over the rod allowing only that much light into the room.
Here is a mock-up of the cardboard blind I wedge into my sliding door in the bedroom. It covers the entire door. The background piece is much larger than shown, leaving room for the sliders to move up and down.
The extra piece can be velcroed on if I need to narrow the sunny space. It could be made much taller than shown for a long narrow slit of light like I would get with my vertical blinds.
You can purchase bounce screens at a photo shop. They are metalic silver and simply used to bounce light back into the set-up wherever it is wanted. I use it to lighten the shadows. You can also use white paper or if you wish colour introduced, the appropriate colored paper (or mat board). When the sun hits the colour it bounces it back into the set-up and/or is picked up in shiny surfaces.
Please let me know if you have any further questions. Cheers. Di