Description: It's a piece of clear acrylic 17" x 39" x 1/4" that sits on the back shelf (inches are used throughout for metric-impaired types, and because plastic shops still use it, curse them). The corners are curved so you can put the top up while it's in place. It clears the top by about an inch along the upper edge, and you can still zip up the window with the windblocker in place.

It's fastened at the bottom by three straps (1 1/2" long) with snaps on them which attach to the same snaps as the top boot. Another two straps go around the seatbacks to hold it up. These straps are made of webbing, salvaged from a knapsack and an old camera strap. You can also buy it in surplus stores or hardware stores. There are Fastec fasteners (like on lifejackets and knapsacks) on these straps to make them easy to attach.

I've learned a few things about getting a better finish. A band saw makes cutting that curve so much easier! A Dremel tool is very handy for cutting the slits for the straps to go through - I had to file the first ones down from drilled holes - not fun. Keep the paper on the plexiglas until you're finished - it keeps the plexiglas from scratching and your drill bits from slipping. "Flashing" the edges helps too - with a propane torch, run a flame along the cut edge of the plastic. This melts the plexiglas slightly and it gives a transparent edge. Don't overdo it, though!

I used to cut two small slots (1/2"x1/2") in the sides to allow me to fold it flat on the rear shelf without undoing the lower straps (this only works on the 90-93 cars). The slots are needed to clear the tenax fasteners for the top boot. This appears to be a weak spot in the design, though, and I don't do it anymore. The top will also go down with the screen sitting underneath on the rear shelf - it fits perfectly.

Note: The diagram and one of the above pictures are of the prototype. I've made a few changes in the design since then, however. The two side notches are gone, as they provided a weak point that seemed to be most affected during shipping. They're not any use to you if you have a 1.8L car anyhow, thanks to the cockpit brace.

I also cut down to two straps at the bottom, eliminating the middle one. There just wasn't enough pressure on them to warrant all three. Now the boot can snap on more places.

Time: I made several of dozen by hand- one for myself, and piles for other Miata.netters. It takes me about 3 hours to finish one to my satisfaction, including all of the straps, snaps, and flashing the edges.
If you get a piece of plastic the right size, all you need to do is cut the large curves out of the two corners. I did it by hand with a coping saw, then used a file to smooth the edges a bit before buffing it. I then drilled holes so I could attach the bottom straps with bolts, and put slots in so the upper straps could wrap around the seatbacks. The snaps are really easy to put in - some might say they were a snap (but that would be too corny, so I won't say it).
I made my prototype out of Lexan, but it's not as well suited to the job as Plexiglas so I've used that for the others. It does have one advantage, though - it's bulletproof! James Bond may drive a Z3, but I have the coolest car.

Tools: Minimum tools (I used these for the first two): coping saw, a drill, a hammer (for attaching the dome snaps) and a file. Well, okay. I lied. I used a brick instead of a hammer. Hey, I didn't have a hammer, okay?
For better looking work, I use a bandsaw, a Dremel tool with a tiny router bit, the drill, a real hammer, a shoemaker and a propane torch - not all at the same time, however.

Results: I've been on many long trips with the windblocker in place. Both were in temperatures that would have been too cold otherwise. It's wonderful - the buffeting is greatly reduced, and with the windows up, it's very calm in the cockpit. It's quieter, too, so I can hear the stereo much better. It makes long highway trips much better, and I can drive in freezing temperatures (literally) without discomfort.
Drawbacks - well, I did have to make it myself :)

Further Customisation: I engraved Miata in Miata script between the seatbacks, and stuck on my email address and stickers from: Dutch MX-5 club (reflective!), UK MX-5 Owner's club, Washington Sports Car Club, Windy City chapter, SAMOA, Miami Valley Miata Club, MX-5 Owners Germany, The Underground Miata Network, Singapore Miata Club, MCA 5th anniversary, MX-5 Koln, Silver Arrow, and of course, Miata.net.

If you've got any questions, please contact me.