The 5ft deep trench is more than 80ft long and was built in just 15 days by the dedicated team at the fort.
Gary Baines, chairman and founder of the Friends of Shoreham Fort, said: “It was a lot of hard work, but it was fun at the same time.”
The idea for a memorial trench had come some time ago.
Mr Baines was inspired by research done by his father, who wrote a book titled ‘The Day Sussex Died’, and started the Royal Sussex Living History Group.
After receiving funding from organisations including the Heritage Lottery Fund, fort volunteers were hoping the building process could involve people from the wider community.
But the pandemic meant just a select team could work at the site.
Construction was completed using 42 tonnes of sand, 294 bags of cement and 2,500 sandbags.
Mr Baines filmed the progress each day and uploaded clips to the Fort’s Facebook page as video blogs.
He thanked contractor Goldstone Services and praised all the volunteers for their hard work.
“The people down there just make it fantastic, they’re such a great team. We have a little family down there,” he said.
During the First World War, soldiers would come to the South Downs and to Shoreham to train how to build trenches, and also to practice fighting in them.
“A hundred-thousand troops were being trained in the area at any one time,” he said. “Over the years of the war, that’s a lot of people going through Shoreham.”
This was all the more staggering considering the population of Shoreham at the time was just 4,800.
“It would have been phenomenal,” Mr Baines said.
The trench will be closed off to the public until April to give the structure time to set.
But next year Mr Baines said residents and schools will be invited to visit.
“It’s all about teaching through experience,” he said. “Education was always at the forefront of our minds.
“The kids learn so much more by doing the things the soldiers would have done then just reading about it or looking at pictures.”
Mr Baines recently announced that the Remembrance service at the fort has been cancelled this year, due to the ongoing pandemic.
He said it made him all the more pleased the trench was finally complete.
“It’s a huge honour that we’ve been able to do this in a year when we haven’t been able to get together to remember these men,” he said. “We are really proud.”