Summary:  These 7 steps are the basic stages of replacing the plastic rear window in a 1990 - 1997 Mazda Miata.  (Mine is a 1990). 

Time:  4 hours

I was fortunate to have my father in law helping me with this project.  There are many steps where a second person is helpful, but I don't think a second person is absolutely necessary.  Please read and think through these steps before doing the project.  Your situation may be slightly different than mine at certain points, requiring a change in strategy.  Please feel free to leave a comment on the Visitor Comments page.

Step 1:  Setup a platform on the back ledge of the Miata.

Take the 2' X 4' piece of plywood and cut it down to 24'' X 39''.  You should measure this to make sure it fits on the ledge of your Miata.

Next fit the piece on the ledge in the Miata.  You will notice that the corners hit against the back ledge and don't allow the plywood to push all the way back.  Cut 45 degree angles off of the corners to make a better fit.

The side with the 45 degree angles cut out pushes to the back of the Miata.

Now use something like wood blocks or boxes to raise the plywood up so that the old window can rest on it laying flat.  You can see the wood blocks underneath the plywood in the picture below.  Also notice the 45 degree cutouts.

The next picture shows the plywood completely in place with the window laying on top.

Step 2:  Cut out the old window
Use a utility knife to carefully cut out the old window.  Take your time on this part because the old window will be the template for the new window. 

Step 3:  Cut out new window
Clamp the old window on top of the Duraplex.

Use a Sharpie to mark a perimeter around the old window with 3/4 inch extra.  Depending on how you cut the old window out, you may want to adjust the 3/4 inch overhang.  A compass might work well for this step, although we just used a ruler.

Place the Styrofoam on a flat surface and the Duraplex on top of the Styrofoam.  Using a jigsaw with a blade with 10 teeth per inch, cut out the window from the Duraplex.  The point of the Styrofoam is that the Duraplex can firmly rest on it while the jigsaw blade cuts both the Duraplex and the Styrofoam.  I learned this technique from watching a youtube video produced by TAP plastics.

Here is the new window.

Step 4: Prepare window for installation.
Peel back some of the protective plastic coating from the new window and use the sponge sander to sand the 3/4 inch edge that will overlap and adhere to the canvas top.  This will hopefully result in a better bond.  Only sand one side and make sure it is the same orientation as the old window.  In other words, even though the new window should be symmetric, if it isn't, it is probably a good idea to not flip the new window over.  Also, use the sponge sander to remove aberrations from cutting with the jigsaw.

Step 5:  Prepare Convertible Top for New Window Installation
Clamp the canvas down to the plywood platform.  Make sure to stretch this out evenly.

Lay the new window in place (no adhesive yet).  Use masking tape to mask around the window.  The more and better you mask, the less glue will make a mess on your convertible top.
Use masking tape to also mask the plywood underneath.  If the adhesive bonds to that, it could be very difficult to get it off.

Practice setting the window in place.  Use duct tape to old the new window.  I found it useful to plant my elbows on the back of my car and lower the new window into place with the bottom edge touching first.  It is extremely helpful to have someone else available as a guide because you won't be able to see everything. Note that the picture below is just a practice run. 

Step 6:  Install the New Window
Vacuum the convertible top and the new window to remove dust, shavings, etc.  Lay a bead of Goop Marine adhesive to both the new window and the convertible top.  The directions on the bottle say to adhere them together in 2 to 10 minutes.  I found this to plenty of time to apply the adhesive, take pictures, and lay the window in place.

Lay the new window in place.  Remember to practice first, and have someone help.  Press down firmly on the bead of glue to get a strong adhesive and a good seal. 

The Goop Marine Adhesive takes 24 hours to cure.  I used a piece of plywood and something heavy (my toolbox) to apply pressure for the 24 hours. 

Step 7:  Cure and Clean Up
Let the glue cure for at least 24 hours (or follow the direction on the glue you use)  and then take the masking tape off.  Clean the excess glue off and enjoy your new functional transparent window. See more photos here.