I’ve been thinking about adding a windscreen to the DF XL. IntercityBike sells one that is attached using velcro. It is a simple shape that sort of fits any velomobile. I believe that it is this one sold for Quests:
I’m sure this works fine but I don’t like the way that it fits nor the use of velcro, which remains stuck to the body even if the windscreen is not in use.
So I came up with a shape specifically to fit the DF XL. Instead of using velcro I’m using magnets. This is a cleaner and more secure way to attach the windscreen. Here are some photos:
It is made with 0.030″ Lexan so it’s pretty light and flexible. It does get in the way a bit getting in and out. It turns out not to be a problem since it just pops on and off so easily with the magnet mounting approach. There are magnets glued to the windscreen and magnets stuck on the inside of the body. I just have these body magnets taped for now.
The windscreen does seem to add some speed, maybe 1-2mph at the high end, but that’s just a guess. You do look through it while riding, so scratches may become a problem. It can get warmer in the cockpit since it reduces air flow into the cockpit. In that case, it’s easy to just pop it off and stash it while riding.
You can download a copy of the template here…
Here is a larger, wrap-around version that I’ve been considering. This one will take a much larger piece of Lexan.
Laid Back Log 
I think looking through the lexan is a problem not only when scratched but also rain-drops and low-sun position blinds me. Perhaps a small spoiler an inch high already deflects wind enough to provide shelter. Transparency only in the middle section so you the depression of front of DF can be used to see through. I am riding with the summer-race hood now, and am trying to cut the lexan away as much as possible. I like the way your screen attaches and fits the mould neatly.
I agree that the Lexan is not ideal for the reasons that you stated. I use the same Lexan for the visor on my hood. At least it’s cheap and easy to cut so I can replace it as needed. Let me know if you find a better material to try.