The plants would probably get enough light from the reflection off the deck to use a semipermanent screen. If you have a Big Lots in your area look at it for reasonable trellis type material that can be modified for your use.

Your pots maybe becoming so warm that they cook the roots. Try making a rolling box with a cover for seating to put in front of your pots. I am talking about the pots only not the plant foliage. Anything attractive to shade the pots. Many people that grow in pots forget that the pots also make a nice steam cooker when you add water.

Tomatoes love heat. Peppers stop producing above about 85 degrees. There are plastic fiberous cloth that can be used as shading. The site below offers many shading options as they also have many greenhouse supplies. Clay pots (unglazed) seem to reduce the heat in the soil thats in them. They also can be sprayed with water and this will be absorbed into the clay.

Here is a link that might be useful: Charleys Greenhouse supplies

I live in the subtropics, where summer temps frequently get to 35C and above. Most of my herbs do extremely well in that sort of heat - with high humidity as well.

I think your main problem is the size of your pots. If you have a range of herbs on a bookshelf-sized planter stand, those pots would have to be quite small - far too small for most herbs. Of course they'll heat up quickly, and also lose moisture very quickly.

I suggest putting your plants in the largest pots you can manage, and make them terracotta pots, which 'breathe'. As the water evaporates through them, they create a cooling effect - much like sweating in humans. Mulch the tops of the pots thickly - human or pet hair is excellent, but anything from stones to straw will do the trick (I prefer using organic mulches because it helps keep the soil in good nick.)

If your deck is up high, as you say, you're probably getting a fair bit of wind up there. That will be more damaging to your plants than the heat. Get some wind-tolerant mature plants or screens and place them so as to protect your baby plants.

As for a shade cover - make a sort of 'table' with 4 tall legs on castors or wheels. The top of the 'table' can be of shade-cloth attached to the framework. See if your local friendly plumber has some metal pipe to spare, or make it of wood or PVC piping. You'll need to put 'brakes' on each corner of it so it doesn't move around by itself in the wind - think the sort of brakes that go on supermarket trolleys and babies' prams. Perhaps have just 2 of the legs with wheels so you can tip and push.

Or get one of those sail decking covers that can be put up and taken down easily.

http://images.google.com.au/imgres?imgurl=http://www.jhc.co.nz/galleries/493350c3af86c/deck_sail.jpg&imgrefurl=http://www.jhc.co.nz/gallery_page.php%3Fgallery_id%3D493350c3af86c&usg=__D2SCAkqCeAu8-Lv4uy-W0h-AgnE=&h=480&w=640&sz=97&hl=en&start=91&tbnid=lDc_l12OXjypEM:&tbnh=103&tbnw=137&prev=/images%3Fq%3Dsail%2Bdeck%2Bcovers%26gbv%3D2%26ndsp%3D20%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-GB:official%26sa%3DN%26start%3D80

Here is a link that might be useful: