You definately do not want to build a bracket on the side of the trailing arms for the air spring lower support.
The design concept behind GM using an I-beam construction for the trailing arms is that an I-beam has very little torsional resistance. Remember the trailing arms mount to the rear axle which articulates, and it also mounts to the crossmember, which is fixed. Therefore when 1 rear tire is goes up, and the other rear tire goes down that twists the I-beam. The rubber bushings at the crossmember take some of the torsional movement.... unless you are using poly bushings... then almost all the twist goes into the trailing arms.
Good quality aftermarket trailing arms use some type of spherical heims joint such as the Sphon spherical joint shown below. This allows these much more torsionally resistant trailing arm to have very little torsion themselves (such as using a tube or box structure) yet the joint itself instead of the trailing arms member takes up the twisting action.
Hope that helps.
John