Create your own bumper banner with nothing more than paper and flour!
1 part flour mixed into 3-4 parts of boiling water makes an excellent glue suitable for bonding paper to metal or painted surface. It forms a strong bond but can be removed by soaking in hot water and rubbing.
Several strips of paper can be cut lengthwise into 3-4 inch wide strips to be pasted end-to-end to form a long banner (overlap them a little to reduce chance of edges lifting up).
You can use a laser or inkjet printer to print your message and graphics, or you can just draw by hand with markers or paint brush.
Regular printer paper works fine but photo paper (i.e. 32 pound weight) is a little hardier.
Of course, a paper banner will start to look shabby after a few rains, but it's a simple matter to make a new banner, perhaps with a fresh, timely new message. You can just layer the new banner on top of old.
*** Your car's clear coat finish under the banner may become dulled, especially if it has been damp for a long time. I've not yet removed my banner to find out the extent to which this can happen. I hope that if the finish is affected a fresh coat of wax after banner removal will make it look like new again. If you have experience with this, please send a note to me at jim@jhall.name to share it.
If you're concerned about possible degradation of your paint finish, you want rain resistance, or you want to avoid hassle of cutting and pasting, consider buying a large removable paint-safe sticker instead. It only costs $11! See "Buy a sticker" for more info.