Hi,
I haven't seen an 'X' brace yet for an M2. Just about everything else though. Corksport has some. Ultraracing has braces for just about every available bolt. Autoexe has some.
The intent is to stiffen the chassis so that steering inputs are crisper. A full cage adds safety to the stiffness but I doubt that a cross brace would help much there.
Should be easy enough to fab. I'd start with a commercial bar like Corksports that ties the rear strut tops together. With some rod ends (rose joints) and tubing, go from the tops of the struts to the seat belt mounts on the opposite side.
John