Got an old inner tube?
Cut a 1.5" cross section of it, making a rubber band of sorts.
Run this, top to bottom, on the outside of your corner.
Set your triangles in place...
Place a 2x4x about 24" (longer than your cabinet by about three inches.) against the edges of your triangles.
Loop the "rubber band" over one end of the 2x4.
Stretch the other end of the "rubber band" hard to go along the outside of the corner to finally loop over the other end of the 2x4.
Tap your triangles (now firmly held in place by the "rubber band") into perfect position.
Scribe a pencil line on the cabinet along the edges of the triangles.
Lift the triangles away a little so you can get glue in between the scribe marks.
Run your four beads of glue.
Now carefully lift each triangle back into the previous perfect position. (Yes, you can relieve tension by slightly lifting the 2x4 also.)
Check for square, and you're done... wait for glue to dry.
You MIGHT be able to simply use an old bicycle inner tube without even cutting it.... though you'll probably have to tie a knot in it to get the correct snug length.
EDIT: I just saw your drawing... Hummm you don't HAVE a corner behind this, do you? Well, that DOES present a challenge. Maybe TWO inner tubes from opposing 90° directions, hooking over the same 2x4?