hey Anthony. I had previously got some 5mm cubes, which were way too big.
I got both rods and cubes in 3mm this time and, as you can see, they are a lovely neat fit between the rails, and the kadees seem to work well with them!
I have a larger amount coming on a VERY slow boat from china - but got a batch from this place
http://www.ebay.com/itm/140814055073?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
And I'd recommend them!