The two spacers of this Thing serve to combine a pair of OpenBeams and OpenRails into a 15x40mm rail system.

I printed at 0.2 layer height, in ABS, with 100% fill.

You can adjust the length of the blocks in the OpenSCAD code, but 20mm looked about right. I use one pair of blocks at every other OpenRail screw slot. The top block is sized to take a pair of M3x10 screws and nuts; the bottom block takes a pair of M3x5 .

The bottom block has a removable support across the slot where the nut is placed. It rips off easily. Once that's removed, with a pair of needle nosed pliers and a bit of patience, you can place a nut in the slot and it will stay there while you insert the block onto the beams. Inserting one point of the nut, then twisting the other side down to seat it works best (see pic).

The OpenSCAD code incorporates the code from http://www.thingiverse.com/thing:45913 in it's entirety. Uncommenting the indicated lines in the code will display OpenBeams and OpenRails in proper relation to the OpenBeamRailSlide blocks.

In the last photo, it's shown with a printed MakerSlide Lens Carriage http://www.thingiverse.com/thing:12512 but this was just done for testing the slide; the printed plate is not rigid enough for practical use.