In in the context of folding bicycles, extenders such as this can serve the same purpose of a stem.

Although they aren't usually found in a large variety of lengths, the way that "road bike" stems are.

That one is by Litepro. I believe its length is 50mm center-to-center.

They come in different clamp diameters (26mm and 31mm), so, make sure it matches the handlebars you plan to use.

As for a stiffer handlepost, the biggest improvement comes by limiting your handlepost choices to ones that don't have any height adjustment mechanism. The telescoping mechanism adds a lot of flex.