Cooling the greenhouse

Don't know about you guys, but where I live, it's been scorching for 3 days now.

I have a few plants still in there that have been grown from seed in the little cardboard pots (I went totally overboard and literally haven't got the room to re-pot every seed I planted) ... Anyway, it seems no matter how often I water these, they dry out and now the plants look very limp and unhealthy 

To be honest, I don't think I'll be using these cardboard pots again as the seeds sowed in plastic trays seemed to be grow much bigger, stronger and quicker!

But, for future ref, any tips on keeping the greenhouse cool? (It's only a small, plastic zip up mini greenhouse)

As always, many thanks

Char