In the absence of better suggestions from others, it seems to me an achievable solution is to buy a 25mm straight steel or alloy tube (something like but probably not exactly
this) and a
saddle clamp. These saddle clamps are usually not very fussy about tube size but yours should have a lip (i.e. not like
this one) to ensure it doesn't slip down if you are using a straight tube.
The tricky bit is to size a tube that is not too heavy yet stout/safe enough to withstand the huge bending moment such a long post will experience at the clamp on the frame. Obviously the rider's weight is an important determinant. For your reference a quick sample from my bike bits bin finds the wall thickness of an alloy post I have to be 3mm, while a steel one is 1.5mm. But they are your typical 350mm length posts. Without doing the maths (but since doubling the length doubles the bending moment) I would have thought as a starting point the wall thickness of your post would need to double these.