forget stitching.
use one camera. make sure it’s WITHOUT any image stabilization. pick a fixed zoom and focus. calibrate it well. good reprojection error is necessary but not sufficient.
make sure the arucos are flat, not just flappy sheets of paper. one aruco in the picture is enough. prepare a series of them in various sizes so he can choose the largest to fit in the picture.
maybe get a tripod for the camera and take two pictures, one for the outline, one for the aruco, so it doesn’t have to be entirely within the shape. it still has to be in the same plane as the shape though.