Once you have four frames completed, you can put them together. Using a 2×4, evenly space the frames so they are 12” on center (ie, measuring from the center of one board to the center of the other board) so the width is 4’ (a normal piece of plywood is 4’, less cutting).
Do this on all four angles inside the vault box so the finished product looks like that.
When you are assembling the vault box, try to make the frames as straight as possible. They should be 90 degrees to the ground.