"They're just big boys' toys. There's a very small market in New Zealand but it keeps one or two of us going," he said.
Mr Cowper competes in four wheel driving and rally driving, trains others in four wheel driving and gets paid to take wealthy corporate people for thrilling offroad rides.
He was brought up on a farm and except for three years working in London as a welder he has lived all his life in Turakina.
When he was about 21 he and his father started buying Suzuki 4WD vehicles, pulling them apart and selling the parts all over New Zealand.
"The business was quite good until Trade Me came along," he said.
Then Mr Cowper built his first 4WD competition vehicle, to his own design, and competed in it. There was a lot of interest from other competitors and he started building more, of various styles, to order.
"I have built about 10 the same since then, 10 in nine years. It takes about seven to eight months to build one."
The finished vehicles are sent off to owners in places such as Whakatane, Auckland and New Plymouth. Two months ago, Mr Cowper took on a full-time assistant, Phil Shailer.
The two are now working on a four-seater 4WD vehicle ordered by Wanaka Adventures - the first of its kind in the country.
"It will hold three customers and scare them with extreme 4WD rides, something like the Shotover Jet but on land."
The two also build 4WD farm vehicles and modify and repair other kinds of 4WD vehicles in a large Turakina workshop.
There is a tourist side to the business too.
Mr Cowper works part-time as a driver for Wellington business Boomrock, which entertains corporate people at three venues in coastal isolation near the city.
All-terrain four wheel driving is one of the activities for visitors who are adrenalin junkies, along with rally car racing, hot laps, archery, knife throwing, extreme golf and claybird shooting.
"It's a huge place overlooking the cliffs on the coast of Johnsonville.
"People fly in in helicopters and I take all the punters for rides."
His 4WD vehicles roll sometimes when he's competing in them, but he said they were very safe for tourists.
Training farm workers and 4WD competitors is yet another side of his business.
He has some development plans ahead, and said the corporate rides and building 4WD farm vehicles were two facets that could be grown.
"The sport has become a business. The hobby becomes less and less important and the business gets more important."