I happen to have a sudden surplus of 2 liter bottles.
And suddenly, while wondering where to store them until next year, I saw The Light !
Take six or seven 2 liter bottles. Group them around one in the center so you have a circle of bottles.
Duct tape the group together at half-way up.
Now pull out the center bottle so you have a kind of 'donut'.
Put in the garden over a new tomato transplant. Fill with water from your hose.
Cost, 10 cents deposit each in Michigan, plus duct tape.
Plants will get some light from through the 2 liter bottles and eventually grow above them.
The heat of the sun will warm the water in the bottles.
Six bottles times two liters is 12 liters.
That's three gallons, roughly.
One BTU is the heat stored in one gallon of water by one degree Farenheit.
If you can store twenty degrees of warmth during the day, that gets you 60 BTU's of heat stored for the cool nights.
Throwing a cap over this setup would conserve that warmth to deter fr*st.
And it's not unreasonable that you could store more heat in the water than that.
Paint the bottles on the North side of the plant black to store more heat. Add a pinch of salt to the water to push the heat storage a bit.
You might get up to 120 BTUs stored in the bottles, ideally.
Enough to last through a 38 degree night. Maybe.