I got myself a couple of those spacer blocks http://openbuildspartstore.com/spacer-block-v-slot/ and attached 2 wheels to each, and fit it to a gantry plate. I quickly realized that on the side with the eccentric spacers all is well, but on the other side, since its an identical block, the normal spacers dont fit inside the larger holes like the eccentric spacers, so there's a loose fit. Is this a good design? It seems to me there should be two different spacer block versions, one with enlarged holes for the eccentric spacers, and one with 5mm holes that fit the wheel axils. Or maybe spacers that have a part that fills in the larger holes?