The MacInnes Stretcher, invented by mountaineer Hamish MacInnes, is being made at a workshop in Inverness.
However, 85-year-old Dr MacInnes' and engineer Richard Glanville's work on the mark 8 (Mk 8) has been affected by ill-health and family losses.
Development has restarted but with dwindling finances to fund it.
Mountain rescue teams, including Glencoe MRT which Dumfries and Galloway-born Dr MacInnes helped to set up in 1961, have been highlighting an appeal for funds for the project.
The Mk 8 is being designed to be lighter and tougher than previous versions by using a composite, a product made with two or more materials such as carbon fibres.
Mr Glanville is leading the active production of the stretcher because of Dr MacInnes' health. The veteran mountaineer continues to supervise the project.
But over the past two-and-half years, Mr Glanville work on the new stretcher has been affected by his treatment for cancer, his father's death and then becoming the main carer of his mother before she too died.
Production is now under way again at the engineer's workshop in the Highlands, but not without continuing challenges, including Mr Glanville selling his house to raise funds for the project.
The engineer said: "The switch to composites has forced a fundamental rethink of the way the Mk 7's components worked.
"Armed with the experience gained from the Mk 7 and the properties of the advanced composite material, many of the fittings have been total redesigned.
"The end result will be a greatly improved stretcher but inevitably this work has extended the development time."
He added: "The benefits to the rescue teams from the new MacInnes Mk8 rescue stretcher will make the sacrifice well worthwhile."
A new version of a folding stretcher used by mountain rescue teams, the military and the emergency services since the 1960s is being developed.