Tim,
I saw the thermoplanter in the Drs. Foster & Smith Pond Catalog last spring. I bought it for $31.00. I wanted to try it so I could have a tropical night bloomer 'Trudy Slocum' like I grew in FL. It is in a 150 gal. Rubbermaid pond and really seems to be working well. Our normal temps, here in the mountains, in the summer are mid 70's in the day and low 50's to mid 60's at night. 'Trudy Slocum' is practically growing out the 150 gal. pond. I don't have any other lilies in this pond, because 'Trudy Slocum' is so large. I do have the usual parrotfeather, elodea, water lettuce and water hyacinths. Also, have 5 small gold fish and a resident frog.
The lid that you asked about, is not metal, but a thin sheet of plastic insulation material. I, also, wondered if the lily would grow through the hole in this cover, but it certainly did. I have had from 1 to 2 blooms everynight for almost two weeks now. I trimmed off 10 leaves today and the pond is still full of leaves.
I only have the one planter and it runs off of the same electrical source as my pond pump. I haven't noticed any appreciable increase in the electric bill.
I have attached a photo of the 'Trudy Slocum' pond.
Ruth