well i am no rocket scientist, but i would imagine that any size that is smaller than your akg bar will fit for the wooden dowel option... five minutes of your time at a homedepot type place should solve your problem...
maybe another five minutes at the store and 10 at home would build that vert bar.
seriously kid... it dont get easier than that... i know my link has a freaking parts list in it...
(edit = your akg bar probably has at least 3/8" in that center hole, probably 5/8" - 1/4", 3/8", 1/2" and 5/8" are all fairly common sizes... you should be able to get in the range if forced to use metric sizes)