Machine bed accuracy

I am going to build a cnc router, and purchased some thk sr25w bearing blocks to run on sr25 rails.
I found information on the net that an misalignment accuracy of 0.025mm per 200mm maximum was required and really the user should aim to be within half of this maximum tolerance.
I had planned to use 100 x 100mm steel box section and mount the rails to them but there is no way that could be done and achieve the accuracy required unless it was skimmed.
The latest idea is to buy aluminium plate maybe 12 to 15mm thick which is skimmed and mount it to the steel frame.
Has anyone any ideas whether this is the way to go and if it is where do i purchase aluminium or steel machined to a good tolerance?
Thanks in advance for any help.
Colin

Have a look at this http://www.oneoceankayaks.com/madvac/machine_frame.htm