I would imagine you would need to build a skeleton crate if you can't find a box that will fit this windscreen. Unfortunately this shipment will volume by quite a bit.
In addition to booking the flight and delivering the freight to the ramp, you'll also need to clear customs in the US. That means you need to create a commercial invoice and packing list.
I would enlist in the services of a forwarder. Give Schenker at LHR a call. 208 890-8899 and ask to speak with someone in air export.