Hi angven,

Select the first plexiglass box by clicking on it.
Click for the second time on the left or rightside of the box to select the surface.
Repeat that on the other boxes.
Click COPY then PASTE and then click HIDE to hide the copied surfaces.
Delete the boxes and click HIDE to show the copied surfaces.
Zoom out and select the curves and surfaces holding down the left mousebutton.
Click CURVE and then click ISECT.