Re: Is it possible to "make my own mat"?
I visited a dojo which had scavenged four layers of Astroturf (from a sports stadium being remodelled, I believe) and stretched canvas over the top. It was okay, though not as springy as I would have liked. You could also try multiple layers of cheap carpet, again with canvas over the top.
I have four dog-eared salvage tatami in my basement. In their home dojo on a sprung wooden floor they were bearable, but on my concrete floor they feel a lot like...concrete. How bouncy is your deck currently? If it's rather rigid you will want a softer surface to compensate; if it has a nice spring, you can get away with a thinner mat.