Yeahh buddy,I am looking for a good arm for my DSLR cage,Are you looking for an arm or just a way to mount it?
Because if you were just wanting to mount it,I would think a great monitor like a Marshall would definitely have a 1/4" hole in the bottom or back?
You could just screw your tripod screw into the bottom,If it was on the back get a piece of aluminium L BRACKET made from a local metal shop.
But if you were wanting to mount your camera as well as your monitor,just get a flat piece of aluminium,and screw one hole at each end,and one side is to mount your camera and the other to mount your monitor,but I would definately take some measurements before doing it of course!