The hinges I've seen seemed rather expensive, as  was the plastic, and I couldn't really determine which of the plastics was the best kind, not being sure which would be waterproof, hard enough, flexible, etc I thought I might be able to make something cheaper.
    The plastic, which I think is acrylic, came from a caravan window. There are usually two layers, sometimes tinted. This is from an un-tinted outer one. I used that because I knew it would have the right balance between flexibility and rigidity as well as being impervious to water. Some of the scrap windows I saw were strongly tinted, and often had curves built into the edge, they usually have a dome to them as well, so you can chose various geometrics. These are almost flat, coming from the middle of a fairly large window. I'd imagine most caravan dealers would have plenty of broken ones, so don't go cutting bits out of your neighbour's caravan.

    I don't like drilling into the U section alloy wind-shield frame, screwing on the pop-studs was quite enough excitement. This method shown at the top, would grip without threatening the glass. So my first attempt was a detachable clamp made from the alloy strips and would have bolted through the hinge, While working on the above I decided on a complete change of plan. The gripper mechanisms worked, but the gaps between the frame and the hinges were too large, so there was a loss of rigidity and would have been draughty. Rethink time.  If anybody wants to copy this method, I can send them a few of the alloy clips
    I'm using alloy piano hinge, £6.40 per foot on e-bay, stainless might be better, but this was available in short lengths, and I wanted un-drilled lengths as I hadn't decided how many fixing points to use and didn't want a series of unused holes. 
    I fitted the hinge so that it is pop riveted onto  the plastic, the hinge is attached on the inside of the plastic, hopefully it will prove strong enough, although I could add a few more rivets. If I go for a folding windscreen later, then it'll all need a rethink.

    Very careful drilling (through a carefully measured piece of copper pipe) showed a depth of 6mm of depth before hitting laminated glass, so I screwed the hinges straight onto the frame with shortened self tapping screws. I think that looks tidier and being fixed directly onto the screen, it's a fair bit stronger, I'll probably round off the corners later, and put some kind of latch or clamp to stop them from flapping. Failing that I'll use a couple of the new high power magnets.

    You might note in the second picture, a wire frame mounted on the door, that's the beginnings of a side window, more on that later. In the mean-time, I'm looking for a new side mirror. I never liked the rather modern one that was just in front of the top of the door, and the driver's new quarter-light wouldn't open with it there. Europa spares have some, but I'll not go for that just yet, as I'm hoping to find a way of putting these ones on, probably from a motorcycle and perfect if only I could find tee nuts for the very fine 10mm thread, It's that or some kind of bracket.
Ist method.Rejected.
2nd method. Working.
Tee nut