Someone else had a same request recently. Fix:
http://www.dahon.com/forum/index.php?showtopic=6443

Also another fix:
http://www.dahon.com/forum/index.php?showtopic=5457

Also, the seatpost is 34mm wide (IIRC), while items like http://www.amazon.com/gp/product/B000FI6WSG have:


Click for full size - Uploaded with plasq's Skitch

There are other seatpost racks which use bolts, for which you need longer bolts for the thicker seatpost.

Add: Depending on where you are striking, you could try setting the rack bag rearwards a little bit; although you probably tried that already.