THE circular ''roof'' of the Eastern Harbour Crossing's Quarry Bay entrance is being converted into a parking lot for dustbin lorries and other government vehicles. The 6,600-square-metre site, about 200 metres from an adjacent secondary school, will provide parking spaces for 90 Urban Services Department vehicles, including 40 refuse trucks. The busy hours will be around 6.30 am, when most refuse trucks go out to collect rubbish, and around 11 pm, when they return. An access road will be built behind the Hong Kong Funeral Home in Java Road to the one-storey covered car park. The department believes the refuse vehicles will not be a nuisance to Quarry Bay residents. Work at the site is scheduled to be finished by February, a department spokesman said. She said the car park would be temporary and would be closed by late 1996 when a permanent car park at Sheung On Street, Chai Wan, is completed.