I've managed to get over my stupidly large gantry design by totalling up the cost of the aluminium plate and extrusion and nearly passing out. My new version 5 design uses KJN 45x90 profile like a lot of builds I see around these parts. What I'm struggling with now though is how to mount the SBR20 to the extrusion.
Click image for larger version.  Name: Wobblycogs CNC v5 - Rail Mounting.png  Views: 1425  Size: 16.8 KB  ID: 7414
In the image above I've centred the rail but it doesn't matter where I put it the rail lines up really poorly with the extrusion. Has anyone managed to find a good solution to this? I'm so tempted to just give up and use profiled rails but the extra cost is off putting to say the least.

A couple more shots of the current gantry design, comments welcome.
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The gantry over hangs the back axis bearing a little so that I have space to mount the stepper. The over hanging weight is fairly minimal so I think it should be ok. The stepper will be slotted so that it can move up and down to tension the belt.

I went with the taller C-shaped gantry design because I want a good separation between my Z-axis bearings. The Z travel is about 300mm (more than I was aiming for) and I'm hoping that extra bit of height in the C-shape will help stiffen things up a bit.