First of all I use carbon steel thick wall because it is stronger that the aluminum ones some places sell.
But the biggest reason is because of the varying thickness differences between different makes of flats and rails.
There is no common dimension used by the manufacturers of flats and rails.
    A support that fits one kit may not work in another.  Do not be fooled by the overpriced finished
center sections everyone else has.  You will end up with more rail grinding or too loose a bolt carrier using one of these so called ready to go supports.  I have never bought supports that were the same length. All were Different.

    When you bend a flat the majority of the upper rail is the correct width.  Only the cutouts for the bolt carrier
and trunions need to be fitted.  The distance between the upper rails changes with different size center supports.The thickness of the rails and the flat also changes the size needed for the centersupport.
The correct lenght the blueprint calls for may be too short or too long.

email if you want just a center support without a kit and if you want it threaded.  If you give me the length I can custom make one.