Most people are put off from building a useful sized CNC machine because of the cost of parts. Others use angle iron and skate board bearings for their linear rails. The instructable may look compllicated but it really is not. With a little care, it is possible to make an accurate CNC machine and will enable you to build a decent sized machine.
The two end sections of the base were cut from industrial shelving brackets obtained from my local junk yard to which I have welded cleats for connection of the linear rails. The two (red) intermediate rails are steel channel with steel cleats welded on the end. The overall size of my machine bed is 800mm x 1800mm but of course you may build your machine to whatever (practical) size that you wish.