From a Wood working view, a plain mitre joint offer little or not rigidity in any direction other that the fixing method be t it screws,nails or glue.
To ensure and rigidity in these drection the minimum would be a simple halving joint.Beyond that a dovetail joint would be better, but more expensive and time consumming to make. Thus, I suspect that a simple halving joint with maybe a dowel or "treenail" fixing would be the prefered method.