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.

There are break away tabs on each end of the bottom about 1 CM deep, clean it and its support up to the cross beam, but leave the rest of the support intact.

If your printer has curling, like mine does, it is high enough that the curl should be worked out by the time it reaches the cross beam.

If you find it doesnt 'fit' file it down, or build it up with scrap abs and glue it into the hole. If it wont sit 'upright' turn it sideways or upside down.