How about cutting a square of MDF with a circle cut out for the port and slide it from the inside to give extra support at the base of the port inside the box. This way you have the base of the port passing through 1.5" of mdf instead of .75". That combined with liquid nails and a couple screws should hold it steady. If you already braced the inside ends of the PVC just cut the circle in half to make 2 arches and "clamp" them around on the inside base of the port, then glue and screw.
Hope this is clear