While this is an older thread, I am reviving it to post some pics of my rock guard rebuild--with much thanks to Jim for showing that it could be done! Judging from his photos, my workmanship is not to Jim's standards but I was in a hurry get on the road last summer (and also had to build a new battery box).
I used 1/8 inch polycarbonate--shop carefully as I found a sign shop to be double the price of a plastics dealer! As noted in a previous post, the frame is very wobbly without the plastic, and they rely on each other for strength. I didn't have a rivet puller when I started, so I went to the 'big orange box' for stainless screws and nuts. Yikes, retail stainless is such a ripoff, so lesson learned. I then bought the rivet puller as I was finishing this project.
I trial fitted the polycarbonate into the frame and drilled and countersunk the holes (for the screws). Then I removed the plastic from the frame to clean out the shavings. That ended up being a lot of work, as everything had to be lined-up and clamped again. In the end, it worked out well. I flipped the brace bracket around as Jim suggested, and had to make stand-offs for the bottom of the frame. I used my new rivet puller to attach those!
While trying to bend the stand-offs in the vise, they kept cracking at the bend, and I ran short of aluminum flat bar, so I had to cobble together one of the stand-offs (note the double rivets in one of the photos). I wasn't able to put as much curve in the dividers to allow for brace clearance as Jim did, so my stand-offs had to be a bit deeper to keep the braces away from the window frames when closed.
I hope someone else can benefit from the photos--good luck. And thanks Jim!!