Sorry to jump onto this thread but I was looking at the same thing.....

I was thinking of using c-beam with 4x2x1.5mm box section with welded ends in each beam and using 2x of them. So the beam would be basically 80x80mm all bolted together.

My other thought was maybe 3x 8020 v slot bolted together.

My design was also to use 15mm aluminium to keep the weight down but I may consider steel now....

I am making a hybrid router plasma, well designing atm so I need speed too...

I am too designing with a vslot aluminium rails and wheels but am considering linear rails now on steal also...

Budget is a massive consideration too.

Regards

Mike.

Ps I posted the same on openbuilds site...