This is an alternative to the Canadian truth truck.
Images were prepared for a pair of lightweight magnetic waterproof detachable signs from signs.com.

The left and right BASELINE ( maximum size) image files are: 10512x6832 and 6912x6832 pixels.
In standard 300 pixels per inch print size, that is about 109.5x71.2 and 72x71.2 inches.
These files can be scaled to any desired values, but affordable signs.com signs are limited to 38 inches of width.
This means the largest affordable magnetic 911 sign possible from signs.com is 38x24.8 inches.
These files can be downloaded from the baseline left and baseline right icons below.

As most modern cars do not have doors that are complete metal rectangles, fitting can be tricky.
Panel trucks would accomodate these designs nicely.

The Baseline files were scaled down a bit for a Toyota Prius. The SCALED file sizes are:
9000x5849 and 5917x5849, pixels, which prints out to about 94x61 inches.
Rescaling this into 30x19.6 and 20.6x20.6 inches provides very good resolution
in a physical size that would fit most simple car doors.
You are of course welcome to rescale to any size you prefer.

To download any of the baseline or scaled files below:
left click on the image; a dialog box opens.
Select "save image as"; this should open a file folder location.
Navigate to where you want to save the image, rename if desired, and click "save".

This process has been tested and should work on your system.
The scaled sign size ordered at the signs.com website was 30x19.6 and 20.6x20.6 inches
with 1/4 inch rounded corners
The cost after working out some bugs was about $125, and the workmanship is excellent.

I have no financial involvment with this company.

Baseline Left

Baseline Right

Scaled Left

Scaled Right