Ok I see :)
You can easily have the result but with "box hole" against "cylinder hole" ;)

Replace Rectangular "Plexiglass box" by rectangular Plan

Close all curves

Select all curves lines closed, Extrude them on the longitudinal side by the measure that you want hole (so a little measure)

Select all volumes result and make a Copie /Size

Make boolean Diff between them

Make Boolean Diff Plexi Glass with result above

Extrude Plexi Glass = you have your result wanted : just Fillet if you want a more smooth result ;)