"Big objects are what we do and they're all prototypes," explained John, a fine art graduate who has run his own business in Brentry for 15 years making everything from film and stage-sets to one-off pieces for trade shows.

Picture gallery: Click here to see the gondola taking shape

"I did building work for a while after graduating but I got sick of standing around in the mud with a bad back so I got into freelancing and set up my own workshop," he said.

In 2003 John's company Bad Dog Design made 60 objects for films and television including the 20ft Bafta mask seen on stage at the BBC television ceremony in April.

Polystyrene blocks

Aileen Davies of Cameron Balloons sewed the silver envelope of the record-breaking orbiter

He is also responsible for the arch visitors to The Priory (with Jamie Theakston and Zoe Ball) came through, for a number of Aqua Scutum's window displays and a rather unusual giant flower commissioned for the Eden Project.

"Sadly it (the flower) was taken down because the people at Eden decided it looked too much like a fanny," grinned John.

The Cameron commission threw up its own problems, not least that of size.

"We had to make the model 10% smaller than the original in order to fit it inside a crate for shipping to Germany for the exhibition, explained John.

"If we had made it exactly to scale, the cost of getting it there would have been prohibitive."

Picture gallery 2: Click here to see more of John's models

The process is a mix of sheer hard work and precision engineering. John uses polystyrene blocks which he carves to scale.

"Everything has to be done carefully - we have to watch we don't take too much polystyrene out in the sanding stages or the edges wont fit and believe me 5mm can make all the difference."

Gas cylinder

"We cut the shape of the gondola cylinder using high resistance wires which are like guitar strings that you pass a low volt current through till they get hot.

"Once the basic shape is completed it is coated it with fibreglass and polyester resin then it is flattened using car body filler and bits of wood.

Jon coats the polystyrene model with fibreglass and polyester resin

"Everything is about finish - you do the thing with the finish in mind and it's often the longest part of the process."

After working on the commission for six weeks, the model is ready for shipping to its new home - an exhibition space created inside a converted gas cylinder in Oberhausen.

"The idea was conceived by Mr Dieter Borer of the Swiss Foreign Ministry with the intent to show what can happen in Switzerland other than mountains and chocolate," Wolfgang Voltz the exhibition organiser explained in an email.

Wolfgang will get to see the model of the gondola for the first time on 10 May when it arrives in a packing crate.

"Suddenly it no longer looks like this polystyrene alien thing and I have to say I'm happy with the look," said John.

"It's going to be hung under the spare envelope (made for the second Brietling Orbiter attempt) which is 58m tall so the gondola itself is going to be dwarfed."

Pressurised air

Across the city in Bedminster, the spare 160 ft silver envelope from Breitling Orbiter 2 has been laid out for checking before being sent to the exhibition.

Alan Noble, Cameron's project director is enjoying the feeling of deja vue seeing the envelope again.

"We never expected to see it back here again, it's very exciting," he said.

The envelope from Breitling Orbiter 2 and John's model gondola in situ in the exhibition

"We built Orbiter 3 in 1999 which made the first non-stop flight around the world but the spare envelope which will be part of the exhibition belongs to Orbiter 2.

"We're converting it so it will be filled with pressurised air rather than helium which is very expensive and it'll hang from the top of the gas cylinder which is 110m high."

Daunting task

The record-breaking real gondola is on display at the Smithsonian in Washington in a hall full of aircraft firsts from the Wright Brothers plane, Spirit of St Louis to the Fokker T-2 - the first plane to fly coast to coast without refuelling.

"When Wolfgang Volz (the creative force behind the exhibition) asked for a replica we got in touch with Bad Dog," said Alan.

"There's no pretense; it's not the real thing, but once it's hung under the envelope it will look very realistic.

Gavin Hayles, Cameron's Gas Department Manager is equally excited by the model project.

"This is a different scenario because the balloon was designed to fly around the world using helium, so to be asked to suspend it inside a building was quite a daunting task but I'm looking forward to the challenge," he said.

"After all we like to rise to a challenge at Cameron's!"

The exhibition runs from 27 May until 31 October 2004.