Johnny Rodrigues, chairman of the Zimbabwe Conservation Task Force, said Cecil's head was found in the home of Theo Bronkhorst, one of the co-accused.
Cecil was shot in July by US dentist Walter Palmer. Zimbabwe is seeking his extradition.
The death made headlines around the world, sending Mr Palmer into hiding.
Mr Rodrigues told the BBC the plan had been for the head to be sent to South Africa and then on to the United States where Mr Palmer would be able to claim it.
But it was seized by police on 7 July from the home of Mr Bronkhorst, a guide on the illegal hunt attended by Mr Palmer.
"The police have it all now and they are using it as evidence," said Mr Rodrigues.
"But we are going to try and get the authorities to release so it can be mounted in the Hwange National Park as a memorial," he told the BBC.
"We can put it near the entrance so people can pay tribute to him. We are just waiting for everything to die down."
Only the head and pelt of the animal are left, he said. "The body has been eaten by scavengers and vultures but the head is intact."
Mr Rodrigues said the Task Force would raise the money for Cecil's head to be mounted in a glass case and would ask the Zimbabwe Parks and Wildlife Authority about the plan "once the dust had settled".
"Any tourist should be able to come and see the cause of all this turmoil around the world," he said.
No one was available for comment from the Hwange National Park or police station, where Cecil's remains are reportedly being held.
Mr Rodrigues said he has been subjected to a torrent of abuse after speaking out against hunting following Cecil's death.
"I had to put my phone off. Hunters are using this as a way to get to me, sending insults and remarks by email and by text."
He said he had been accused of taking money from hunts, a claim he denied. "I hate hunting, I don't believe in it," he said.
Cecil's six cubs were being looked after by another lion, Jericho, according to Mr Rodrigues. "We were expecting the worst and for the other male to come in and take the female and kill the cubs. But Jericho is doing a good job.
"They are following him around and he is playing with them. We are monitoring them all the time and everybody is happy."
Cecil was "iconic", Mr Rodrigues said. "He was a tourist attraction and I hope we can get the tourists back."
A Zimbabwe conservation group says it wants the head of Cecil the Lion to be mounted in a case in Hwange National Park, where he was killed last month.