Well, the three guys mentioned in the article creating hand made post stamps were actually put on trial but ultimately exonerated because they did not attempt t

Well, the three guys mentioned in the article creating hand made post stamps were actually put on trial but ultimately exonerated because they did not attempt to imitate any existing or valid stamp, if I recall correctly the point was that there was not forgery involved, and thus there was no crime, it was considered like if you mail a letter without a stamp (or with a stamp with a value lower than the needed one) it is up to the Post Office to notice and ask the postage (or the postage difference) to the sender or to the addressee when it delivers the letter.