Paul Somerville, 21, managed to leave a so-called "cell on wheels" in Maghera in January 2012.
Mr Somerville fell from the back of the moving van close to his home at Church Street, shortly after he was arrested.
He was being taken to Maghaberry prison when the incident occurred.
A doctor from a nearby health centre treated him at the scene before he was transferred to Antrim Area Hospital.
The trainee electrician died in hospital three days after sustaining serious head injuries.
During an inquest hearing last year the man's father said his son was likely attempting to escape.
Police Ombudsman Dr Michael Maguire said two officers who accompanied Mr Somerville failed in their duty to ensure his safety.
The cell door was misaligned with its frame and its latches did not always fully engage, forensic examination showed.
Dr Maguire said: "Given that the same design cell is used widely by other UK police forces, and is still being fitted to new vehicles, the PSNI has said it will share the findings of the case with police across the UK."
The two police officers who accompanied Paul in the van failed in their duty to ensure his safety by failing to ensure the cell door was secure.
Dr Maguire said an examination of the cell found the latches did not always fully engage, even when the door was slammed shut.
He also discovered that a deadlock did not engage unless the key was turned anticlockwise through a full 90 degrees, even though a locking bolt could be seen moving as the key was turned.
Both officers involved in the case said they had seen the deadlock in the cell door engaging after the door was closed and one added she had pulled the door twice to check it was locked.
Dr Maguire said forensic examination showed that the door opened easily when pulled if it had not been properly secured.
Tests showed that even where the door's latches did not engage, it would not open if the deadlock had been fully locked.
The officers were disciplined by the PSNI but appealed, and sanctions were withdrawn.
One issue under consideration was whether the victim fell or jumped to his death.
Forensic evidence indicated that it would have taken a deliberate action by Mr Somerville to open the rear door because it could only be done by pulling a handle.
Two people spoken to by investigators said they had seen a man jumping from the van, but refused to provide formal statements, the ombudsman's office said.
Police had reported a suspected fault with the door when the van was serviced four days before the incident.
The ombudsman said: "The mechanic who did the service recalled that the door had been misaligned and said he had fixed the problem.
"However, the issue was not entered on the vehicle's records as it was not part of its normal service routine."
The PSNI has fully co-operated with the inquiry by the Police Ombudsman, welcomes the findings and is determined that incidents of this nature must not occur again.
Assistant Chief Constable Mark Hamilton
Dr Maguire recommended that cells and other modifications should form part of normal service routines.
His other recommendations have resulted in modifications by the PSNI.
Notices have been attached to cell doors warning officers to check locks are fully engaged, plates have been fitted to prevent doors from being opened from inside and larger viewing panels have been installed to improve monitoring of prisoners from the front of vans.
The man's parents, Desmond and Gwen Somerville, said the PSNI was not duty-bound to act upon the recommendations.
They asked: "When serious human error is involved, who can the public go to for justice?
"Our expectation was that while in police custody Paul would be safely conveyed. He should have been.
"Paul was our much-loved and only son, and we have been left devastated by his untimely death."
Assistant chief constable Mark Hamilton said: "The death of Paul Somerville was first and foremost a tragedy for his family and friends.
"The Police Service of Northern Ireland extends its deepest sympathy to them for their loss.
"The PSNI has already accepted the findings of the Police Ombudsman and has implemented a number of recommendations aimed at improving both the safety and security of members of the public travelling in cell vans."
He said two officers received superintendents' written warnings for their failure to ensure the cell door of their vehicle was securely locked.
These were subsequently overturned on appeal via the PSNI disciplinary procedure.