This is what I use. Another tip I found is that darkening the image helps it show up better. If you are a pleb like me that only uses MS Paint for his image needs, Paint.net can darken your image and it is a free program.
You can also check to see if your printer has an option for printing photo-quality. I believe this uses way more ink though.
I also leave the black borders untouched when I take the ink off with acetone. You can do this by using painter's tape on the edges. This makes it so that if your card size is off by a little or you cut it slightly wrong, there's still black on the background to give you some wiggle room. It's a lot more work to leave the edges untouched, though
Something like this could work, but I have never done it myself and haven't really researched it, so I'm sure you could find something better.
Just remember, a typical printer won't actually put down white ink, it just leaves it blank because the paper is typically white, and this particular product is clear.
buy some of these either online or at your local office supply store. Print out grids for different map projections as needed.
You will have margins, but if you're overlaying a map that you printed, you'll have the same margins anyway.
Found this on amazon: Grafix Ink Jet Adhesive Film, 8-1/2-Inch by 11-Inch, 6-Pack, Clear https://www.amazon.ca/dp/B000S15KDS/ref=cm_sw_r_cp_apa_i_hx7MDbGHR7YK5
Just thinking about how to do it myself. I understand my method is most likely not how this was done but I can see it working still..
A square piece of transparent film, just a bit larger than the mag mount base, placed on your car roof resolves the problem with scratches. Something like this: http://www.amazon.com/Grafix-Adhesive-2-Inch-11-Inch-6-Pack/dp/B000S15KDS
I've been using this stuff, but I highly suggest you avoid it unless you have an inkjet printer.
Most people I know use this method. You could also use this stuff in place of the tape, but I haven't had any personal experience with doing that.
can you get prinable sticker paper like this in Colombia? or maybe like this? you can find them at office supply stores and sometimes craft stores.
on my original DS i cut out a picture i liked from a magazine and and placed the sticky sheet plastic over it to stick it on. looked really cool.
i can take pictures when i get home if it'll help. good luck!