Many of you mentioned Ikea Billy bookcases. Hereby I can confirm that you can fit 3 modulars / 3 baseplates per shelf in them, only using a little trick. :). The trick is, you do not need to fasten the the screws as hard as you can, just loose them a bit. They will still hold well, but it will give you those missing 1-2 millimeters.
IMG_0332 copy by Ivan Kozirev, on Flickr
IMG_0333 copy by Ivan Kozirev, on Flickr
I invented this technique when I wanted to display my space sets in a black Billy bookcase using 3 baseplates next to each other.
IMG_0334 copy by Ivan Kozirev, on Flickr