I also over-winter mine indoors. I use a one of those big plastic flower pots from Walmart. I overwinter papyrus, tropical water lilies and a rush kind of thing too. They are in an unheated sunroom and survived some of the coldest weather we have had in years this January.

I found thru trial and error that water lettuce in a bright window inside in winter will thrive in clear water as long as you have a clear dome over the clear container to keep nearly 100% humidity in. Plants in an open aquarium shrunk and withered but lived but those inside an individual McDonald's ice crème desert cup with the clear dome on and one tiny vent hole in the top lived all winter with no shrinkage and when put outside in very cool mid March weather multiplied and made over 100 plants in about six weeks in an open 30 gallon laundry tub of rain water. If you float a few in a fresh water aquarium the long roots will remove a lot of nitrates from the water and many fish love to spawn in them as their little fry can hide among the fiberous roots.