The Rudder, Elevator, Flaps and Ailerons are easy to set up on flat blocks because of their straight surfaces. The Horizontal Stab is not so easy because of its airfoil shape. Here is a method I have been using that produces consistent results and a very straight Stab. First step is to source 4 aluminum angles of the dimensions shown. Any similar material is suitable.
Step number 2 is to set the supports on your workbench at the proper location, you want the edge of the angle to line up with each spar, this way when you have riveted one side, the rivet head will be over the angle and still produce a level surface.
Step number 3 is to level the angles or support pieces to ensure they are perfectly parallel to each other. Thin aluminum works well, even playing cards would work.
Knowing you have perfectly straight supports, you can flip the structure around during the skinning process and as long as it is contacting the supports, you will know it is straight. I wanted to show the assembly without the skins for clarity, but the idea is to use this throughout the drilling and riveting of the skins to spars and structure. The 3" height allows clearance for clecos on the underside.