PrintRbot Top Joiner - Plastic Cross Beam.

Printrbot Basic, 6 inch bed, 7 ½ inches corner to corner.
It is almost a corner to corner print.

This fits the hex nut hole, which ever model you print.

THIS DEPENDS ON ABS SHRINKAGE, it is too large otherwise.
At 40% fill it fits just right.

It might require abs/acetone gluing to make it structurally sound on your configuration.
I glued mine into the mounts and they are solid with no flex.

The raft serves a purpose. It is additional support.
Use hex infill and hex support.