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I'll second that. Yellow masking tape is not fun to remove. Don't ask me how I know
I just went to my local sign shop. Most will be able to print and cut anything you want. I have peeled the numbers off my car several times inbetween racing seasons. There is a slight residue that can be removed with goo (tar)remover available at any auto parts store. The goo remover also removes the cone scuffs if you hit one during the autocross. Then wax the car and there is no trace.
I ordered some static cling decals for the upcoming 4th of July parade to help spread the word about electrics. I took a 25 mile drive at speeds up to 65 and had no problem with the decals.
By the way if you want some signs like these I can get them to you for $35 plus postage.
If you are looking for some good static cling I strongly suggest bigtexbanners.com
I think static clings are the best when it comes to short term displays on cars