Many options here but take your time ...Shade cloth is the obvious one but be wary of the recoil as the rocks can bounce back into the rear window of the towing vehicle, some ensure it is angled down and this may be enough... Im sure a travelling "grey nomad" out there has the perfect solution for you.

Best solution is a bit of a frame running down each side of the A-frame and some type of material affixed to this (shadecloth probably cheapest but amy need to be replaced after a while which is easy enough).  The fact it forms an arrow of sorts along the A frame should deflect anything to the side and down and prevent rocks coming back towards car.  But note this doesn't stop rocks bouncing back up and hitting nearer the rear of the boat only those flicked up from the car  Peice of rubber hanging down from the rear of the trailer mudguard may help smother any being flicked up by the trailer.

I was thinking something going flat alont the a frame of the trailer like a fine mesh (though something to stand on would be handy)  The chips appear to be further back not right in front of the towball.  I drive a spacecab hilux so the tray would block out a lot of those close up potential chips right at the bow.  Though saying that a small deflector at the front on an angle would be a good idea too.