Sheets and bedding were placed over the sides to prevent her seeing outside, no lights were working inside her bedroom, and a mattress and sheet were also covering the window, the court heard.
Ellie-May was alleged to have been bound to the bed face-down. There were also marks on the child's wrists and ankles, suggesting she had been tied to the "cage", the jury heard.
A post-mortem examination concluded the youngster's death was caused by "forcible restraint by ligatures in a face down position complicated by hyperthermia".
Coyle, of Collins Road, Bamber Bridge, Preston, said the bed had been converted by Hitchcott, of The Fieldings, Fulwood, Preston, with the aim of getting the toddler into a routine of sleeping through the night.
She told the court that Ellie-May, her daughter by ex-boyfriend John Minshull, was her "world". The case was adjourned for sentencing at a later date.
Granting Coyle and Hitchcott bail, judge Mr Justice Dove said: "That is no indication whatsoever as to what the likely sentence might be."
Ellie-May's father John Minshull said: "Ellie-May was a wonderful little girl, she was so special to everyone around her. She was always happy and the best daughter anyone could ask for.
"The fact that she was taken away from us broke our hearts because she was loved and she will be missed deeply."
Brett Gerrity, from the Crown Prosecution Service, said: "This is a tragic case in which a young and innocent child was treated in the worst possible way by those who should have kept her safe from harm.
"During the trial the prosecution proved that, by restraining Ellie-May in her bed, or knowing that she had been restrained in such a way, Lauren Coyle and Reece Hitchcott wilfully contributed to or directly caused her untimely death.
"Throughout the case the defendants denied responsibility for causing her death and blamed each other, but today the jury have found the child's mother and her partner guilty of causing her death and neglecting her.
"I would like to express my sympathy to Ellie-May's family and loved ones."
Detective Chief Inspector Zoe Russo said: "This case has been the most distressing that myself and the team have ever had to deal with.
"To think of the suffering that this little girl was put through by those people that should have loved, cared for and protected her is heart-breaking.
"This was an entirely unnecessary, pointless and avoidable death of a child."
An NSPCC spokesman added: "Coyle and Hitchcott's actions caused Ellie-May pain and anguish that ultimately led to the end of her very short life.
"Looking out for babies and young children, who depend entirely on those caring for them, is something we should all do.
"And the NSPCC is here to help. If you have a concern about a child, call 0808 800 5000 and one of our helpline counsellors will be there to listen."
