There are quite a few pieces of angle iron in the Armstrong cannon. In the original they measure 95x95mm, and are about 15-16mm thick. Also, there is a definite radius between the 90º faces. At my 1:10 scale, the material becomes 9.5mm x 9.5mm, and about 1.5mm thick.
After considering various options, which included using extruded aluminium, and bending some sheet mild steel, I decided on the following solution….
I bought some offcuts of RHS (rectangular section) with 1.5mm steel thickness, and used a bandsaw and milling machine to produce the required dimensions in steel.