Yesterday, someone slipped this sloppily handwritten scrap of paper under my door.
which (without the adverbs and fancy adjectives) essentially conveys this idea:
Someone gave me a handwritten paper.
But you don't have (see below), do you?
[...] a written paper.
A safe alternative:
Someone gave me a piece of paper on which there was handwriting.
which is too long and bland for my purpose.