The most important thing is to make it large enough. For hardening off plants, and perhaps growing one or two ridge cucumbers during the summer, consider a minimum size of 4ft by 3ft. Old windows make ideal tops – two or three laid side by side can make a good-sized frame. For flexibility make two frames instead of one – this will allow you to keep different plants at different temperatures.
Use bricks or thick wood to make long-lasting sides for the frame. Or make the top and sides by covering a wooden framework with thick polythene or rigid plastic sheeting. Either will keep the soil warm almost as well as glass. Line the sides with ceiling tiles or expanded polystyrene sheeting during the winter to help to keep more warmth in.
For good light penetration and to help water drain from the top, make the back wall higher than the front. A frame 6ft deep should have its back wall about 18in high and the front wall 12in high.
Ventilate the frame by sliding the tops to one side, or raise them with bricks or blocks of wood. Hinged tops are more secure in windy weather, but they can be a nuisance because you may want to remove them completely during the summer.