Solid masts that fit your bike were sold by Dahon with your bike. Finding one though will be difficult. They are out there. Part of the search problem is people call them stems, masts, handlebar posts (probably the most accurate and used by Dahon). Save an eBay search Dahon handlebar post, stem, mast. Don't specify Helios.

I would also try contacting Dahon USA. I would have said contact Thor, but he retired.

Helios, due to its lightweight, was a popular bike.