As always, I have tried not to buy anything for this project, but only use recycled objects. I decided on cardboard egg cartons and on plastic fruit boxes from the supermarket. I cut the egg carton to size to fit into the plastic box. Then I covered most of the ventilation holes in the box with selo-tape. The inside of the egg-box I filled with soil and seeds.
It surely isn't pretty, but it is cheap and it works. After less than 7 days on my window sill the tomatoes have already sprouted.