Whilst they have custom made covers you could easily improvise using a poly (or preferably canvas tarp) - lay it across the draw bar tie a rope to each corner of the tarp then pull the bottom ropes and tie them off somewwhere around the mud guards on the trailer, zip tie the centre of the tarp to the winch post top and the then tie the top corners to your boats rails as fat back as possible forming a v shaped stone sheild - it shouldn't rub on the hull and should be resilient enough to absorb and deflect most stones - hope my explanation is clear enough !  Saw plenty of custom made deflectors when we visited the Territory where they were laced to the winch post and tied back to poles extending from the trailer like guide posts and they work great   

What size tarp do you suggest. Not sure how you go about zip tying the centre to the top of the winch post not sure how you mean. Is the tarp underneath the draw bar or on top??

 Head to the local tip, pick up some old stuff for bugger all, buy a heap of cable ties and get creative... Lot less wind resistance than a tarp, I use the same bit of old stuff on my boat for he gnarly limestone and gravel trip up to gnaraloo regularly and it always comes through unscathed... Double it up if your extra paranoid. 

Works well at higher (70-100km) speeds as well

And it sits on top (just) of the draw bar and forms a V running back either side to the guards and the boat rails (the further under you can get the bottom of the fabric to sit the better.   As I said they have custom installed ones in the Territory - I will email our Darwin manager who is a mad fisho and see if I can secure photo

http://www.boatpoint.com.au/boats-for-sale/boatdetails.aspx?R=15572515&Silo=Stock&Vertical=Boat&Ridx=9&eapi=2

Bloody nice boat too - and well priced but there are two photos which clearly show how the stone sheild is installed - hope this assists

Thanks guys lot of good ideas and should set me on the right path. Nice boat sunshine.

If you are going to put a stone deflector on your trailer its also a good idea to put some cardboard(or similar) with sticky tape over your rear windows. Have seen stones get bounced off the deflector on the trailer into the rear windows of wagons before damaging or smashing the glass.

I will probably be setting up the stone protector so it is close to and at a similar angle to the bow much like the below picture. My theory is that with this type of setup it should not deflect back the stones onto my Prado-Hopefully.

https://www.dropbox.com/s/1n8t4iwvvh8yvud/Shade%20Cloth1.jpg

why not a larger set of mud flaps on the car will stop most of the rocks or even a large piece of conveyor belt ?

http://www.ultimate4wdequipment.com.au/d210-81/rock-tamers-floor-mats/

Corflute is corrugated plastic sheeting duct tape to hull and will absorb hits leaving hull untouched available in anything from 3mm up to 15mm usually 2.4mx1.2m