I made myself a telescopic seatpost from aluminum pipe with 2mm wall thickness. Basically some sort of extension of bike seat tube with another seat clamp at the top and frame seat tube clamp at the bottom. Then I inserted into it a normal suspension seatpost for regular bikes. I used Suntour SP NCX 12 which I consider better than ordinary seatpost suspension that you can find for folding bikes, such as Biologic. This Suntour suspension is so good I have it on my four bikes. It has proven itself as very reliable over many years of riding. It works great on both folding 20 inch bike and my touring all terrain bike.