As a practical and cost effective solution, I'd advise using aluminium foil, or tin foil as it's also called. Please realise that it's rather fragile, so it's best to glue it to a plastic foil or something to keep the wind from ripping it apart. It doesn't really matter if cracks get in the foil. Use the most shiny, reflective side of the foil as the outside. This maximises reflection of heat in the form of radiation.
Make your roof wider, so it covers more ground surrounding your crops. If the sun is able to heat up the ground right next to your crops, the heat in the ground will travel to the ground under your crops, and ending up also heating the crops themself. You don't want that.
You can't get or keep it any cooler than the surrounding air, so make sure it's ventilated, to keep temperatures as close as possible to outside air. That's the lowest you'll be able to go. Therefore, don't hermetically insulate your crops like you did in the picture, but enable it to ventilate. No insulation can keep all the heat outside. It will let through more or less heat. Any heat that is let through by the roof is removed by ventilating. If you cover them like in your picture, any heat let through by the roof is instead kept under it.
I'd advise to set up your roof like this. You'll also keep the ground surrounding the crops as cool as possible this way, minimising any heat conducted to the crops.