Steve,
What about fabricating a bed up out of tooling plate, This is high strength alloy that has been ground flat.
One large plate of say 12mm with strips of say 6mm 90mm wide with an 8mm strip on top of that 100 mm wide.
Viewed from the end this will make a tee slot when mated up with one at the side.
CSK Allen screw thru the top two into the bed plate will hold it all together.

This way you get the exact bed layout you require and are not held to any extrusion layout.

Try ASC Metals at Lincoln, they stock tooling plate and will cut it to any size on their big bed saw and it a good cut and square, a bit of draw filing is all that's needed, no machining other than drilling and tapping.

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