I bought a used Dahon CIAO that had a BMX fork put on it. I have since taken it to Guatemala and given it away.
As I recall it was basically a threadless headset and held on my the two bolt clamping of the folding handlebar stem.
Before shipping out I added a device call a head lok to add more assurance like a star fangled nut but it extends all the way thru to the bottom of the fork tube.
If you saw one you could probably make your own.
These are the only pictures I have left of the bike and none of them show how it was done, you may have to innovate.
You might try to adapt a BMX fork from a donor bike.
