Last year i bought a dozen small 5 foot round all plastic solid swimming pools from the dollar store! They were reduced from $12 to a dollar each . I will use them this week to put about 40 flats that need another 2 weeks to mature> The reason I am using them is its nice to put a inch of water to water from the bottom without watering twice a day! They even make a nice planting flower box for the summer or a seed growing area in the shade to start seeds all summer!
+++ I often wondered why people don't use there in ground swimming pools for a winter greenhouse simply by adding a few 50 gallon water drums and a cover to make a solar greenhouse! It should work great right?