In the case of the solar box, the inner structure is constructed using square aluminium profiles.Having already drilled holes in the prepared square aluminum profiles as specified in the "mounting system" technical drawing PDFs (see step 3), this step is about assembling them to form the inner structure of the solar box.
The positions can be seen in the following technical drawings. The profiles can be screwed together with bolts, washers and nuts.
First, connect the 25 mm wide vertical profiles with the 2 long 15 mm wide horizontal profiles, like you can see in the following photos.
Then attach the short horizontal profiles to one of the already connected sides. Finally, connect the two parts.