fortunately my previous cat flap purchase was one with a very similar cutout size, so the job was a comparatively easy. The overall frame size was similar to your 304W, so consequently I'm very pleased with the ensuing result. I must emphasise the aperture should be cut 'exactly' to the required size otherwise the external frame distorts, making the flap somewhat difficult to close properly. However I'm very pleased with the result. Thankyou for such a beautifully engineered facility, our cat has no problem gaining access or egress. Regards, John Pickett. ( order no. 305159 16211 PICKE004)