I followed a similar thought process Wobbly... I've seen folk suggest bolting thick flat bar to the extrusion and then in turn attaching the SBR20 to the flat bar, but IMO that partly negates one of the main benefits of using extrusion... relative ease of assembly.
Also just a quick caveat that the SketchUp 45x90 component... certainly if you are using one of mine, sorry... may not necessarily be accurate enough for this kind of accurate line-up exercise.
Attaching a plate or bar to the extrusion was all I'd come up with as well so at least I'm thinking along the same lines as others. I agree though it seems to defeat the purpose of using extrusion. I'm using the MacTavish 45x90 component. It seems to be right but I'll double check it.

I'm seriously tempted to swap out the extrusion for some box section aluminium with a good thick wall. I can't help feeling that three pieces of box arranged as I currently have the extrusion would be equally strong. I'd almost certainly then fill the middle one with kiln dried sand to damp vibrations. The problem then moves to joining the box section to the gantry sides but that feels like a fairly easy problem - I feel version 6 coming on...

I was looking at the Zapp rails yesterday. The rails aren't to bad price wise it's the carriages that are the killers. I'm trying to convince myself that I'm only even going to build one CNC so I might as well do it right the first time but I just know I'll end up building another :-)

I've found that there are sooooo many tradeoffs in any DIY CNC design.... That it's worthwhile setting aside plenty more time than you initially anticipated ;-)
Never a truer word spoken.