Tired of wrestling with your sunshade? Modify one to fit your car. As a bonus, the better it fits, the more heat it keeps out of your car!
Step 1: Initial fit check
Lay your sunshade out flat across the outside of your windshield to get an idea of what you have to work with.
If you're picking up a new sunshade for this project, get one that covers as much of your windshield as possible when you lay it on the outside. Sunshade manufacturers want to keep their costs down, so they produce the minimum number of models. I have only seen two sizes in the auto parts stores: regular and large. If you read the application notes on the back, you'll see a slew of makes and models allegedly covered. Ignore these suggestions and get the big one: you're going to make it fit, and the extra size will come in handy.
What you see below is my trusty 1987 Toyota Camry (affectionately called "The Death Trap" by coworkers) with a large sunshade. You can see that there's plenty to work with. The sunshade has shiny mylar on top of some sort of bubble-wrap-like material. If you want to keep costs down, and you're willing to do a bit of folding at the end, you can make your own sunshade from a big piece of cardboard.
Windshields tend to curve in at least two dimensions, which makes them tough to size using simple measurements. We need a template to guide our modifications.
Step 2: Start template
Our template is just a piece of paper that reproduces the outline of the windshield. Tape some big pieces of paper together until you've got something that covers your whole windshield. I used butcher paper, but newsprint would work fine if you're careful.
Temporarily hold the paper in place, I was able to use the wipers, and use a single-edged razor blade or utility knife to trim the top edge. Follow along the glass next to the trim at the edge of the windshield. You don't need to press hard, all you need to do is cut through the paper. Once you've got the top edge trimmed, use a few pieces of tape to keep the paper in place while you do the other edges.
Step 3: Finish template
In general, the idea is to trim the paper so that it follows the edge of the glass all the way around, but there may be areas that need special attention. Examine your windshield and note areas where there is glass, but no space for the sunshade inside the car. Trim the template to follow the border where the sunshade can go.
Step 4: Template versus sunshade
Remove the template from the windshield and lay it out next to the sunshade. Note that the sunshade is mostly rectangular, but the template is not. This is why you end up fighting with the sunshade: it's not the same shape as your windshield.
Step 5: Position template
Now place the template on top of the flattened sunshade. Make sure it's centered and the top portion of the template is at the top edge of the sunshade. Move the template around to get the best coverage. Areas where there is paper with no sunshade underneath are going to be gaps in coverage. I favored the bottom edge a little more for two reasons: the sun visors can fill in a bit at the top, and my car has a "lip" at the inside bottom edge that the sunshade can engage for a good fit. Once you've got the template where you want it, pin it down with something heavy to keep it from shifting around.
Step 6: Trace around template
Smoothing the template and sunshade flat, outline the edges with a marker.
Step 7: Trim sunshade
Trim along the lines to get rid of excess material. Scissors worked fine on this sunshade.
Step 8: Try it out
Check the fit. The template should get you close, I didn't have to do any additional trimming. If it can't be smoothed against the inside of your windshield and still wants to buckle, take the sunshade out, trim a little from the edge, and try again. Don't get aggressive on the trimming, if you remove too much a gap will be left for the sun, so take it slow and easy.
Now you should have a sunshade that: fits your vehicle, requires less wrestling, and does a better job of keeping the sun out. Enjoy!