Maybe one of those garden trug things which are about a fiver
https://www.homebase.co.uk/garden-tr...en-42l_p497502 with topsoil in it if you are stuck. The soil would have to not be wet and it would have to be in a cold room where the sun doesn't get to it to heat it up. Insulated with bubble wrap or something round it to keep the temperatures more stable and maybe a sack covering the top and allow some air to get in. Even an unheated room in a house these days is still about 14c though