Any inexpensive ideas, I don't want to move them. Would some sort of shade over the containers make an effective difference in soil temps?

I am growing in a soil similar to mel's mix, in plastic pots.

Here is a link to some pics from my garden, sorry not all the pics awe labeled.

You could bury the container, even doing so 3/4 way down helps tremendously. Or paint them white. Yes putting shade cloth over them so direct light doesn't beat down on them will help tremendously, but that may reduce the light on your tomatoes /shrug. You could loosely wrap the shade cloth around the container itself -- that works fairly well but I've noticed unless the shade cloth is white, darker cloth will tend to absorb the heat and it still radiates from the cloth. Simplest? Just loosely wrap (or lean) some cardboard around it -- ugly though.

One of the best methods is to do pot-in-pot (cache pot) where you put your plastic one in a larger container leaving at least 2" of air space around it. To improve that further, put very large sized bark nuggets in that space. It will keep the inner pot cool in summer and protect it from colder temps in winter. That obviously doesn't work for self-watering containers.

I do a combination of the above ideas. For some I buy those cheap car shades from the dollar store and wrap them around the container. It reflects the light to surrounding plants and keeps the container cool.

In the hottest part of my yard where direct sunlight beats down on my container citrus all day, I am forced to use shade cloth over them all. Many plants stop photosynthesis production when root temps and/or leaf temps reach 100F. They also don't need as much sunlight light as some people think -- but I live in the Central Valley of CA with tons of sunlight.

That's the gist, but there are some other ideas in this forum as the question comes up quite a bit.

Thanks,
DL