Cloches

The words "cloche" and "clock" appear to come from the Latin "clocca",
and refer more to the bell-shape than to the function. The function of a Cloche is to protect plants from cold, heat, or loss of moisture. The function of a clock is to tell you when to turn off your sprinklers : )

A Cloche can be made of any material so long as it serves its intended function. A box, barrel, aquarium,  tin can, or  straw hat would protect the plant, but there are other considerations. Of course, it must be large enough to allow growth of the sheltered plant(s). And not so large that it interferes with other plants in the garden and its ability to be moved about.
 

Here a clear piece of fiberglass is used for a Cloche. In the background is one made of Vinyl House Siding. Ends can be open or covered, depending on the weather conditions.

A cloche left on for an extended period of time (as to protect newly-planted seedlings) must also allow light to enter. And then there is the problem of fungi growth in a closed environment; so ventilation must be considered.

A survey of gardeners indicates that a Free favorite is the gallon-size opaque plastic milk Jug. It can be left on the seedling for a week or two, and will allow adequate light and ventilation, provided the cap is removed.

If you want to protect a bed of plants, you may prefer a  12" high frame with a glass or plastic pane, window, storm window, or storm door fastened to the top. It can have 20" bicycle wheels fastened to one end, and handles on the other end to allow you to flip it and easily roll it to a new location.

If you use aluminum storm windows or doors, you have the advantage of ventilation and light weight. And if you use it with the wheels at the edge of your garden, flipping it end-over-end will remove it with minimal disturbance to the plants. This portable "Cloche" can do anything a Cold Frame can do.