My neighbour's husband brought over a small plastic greenhouse, which I had intended to lie flat, but the design enables me to unzip the door from the top as well as the bottom, avoiding the wicked heat build-up which plagued my earlier greenhouse similar to this one.
We set it up against the wall, where it gets filtered sun most of the day, and I popped the flats on the top shelf. (I decided to take a bath, even though I'd pledged not to do so until transplant time.)
It's 8 C (46 F) outside, but we have a wicked wind. I put a thermometer in the greenhouse, tucked up in the shade of one of the flats, and it's read a steady 18 C - 64 F, which is probably a little cool for tomato seeds but it will probably be close to 20 C - 68 F by afternoon, which is poi-fict!
The one drawback you can't escape with this type of greenhouse is that although it has three generous shelves, the top one shades the lower two, so really only the top shelf is good for seedlings. This means when I get more flats planted I will turn it on its back, so there's no shading. Then it will function more like a cold-frame.