Aspen is ok, but it can be a mess. If you use it make sure that the layer is less than 1/2" thick or else the UTH will be useless (aspen conducts heat very poorly)

I prefer paper based substrates. They are easy to clean, cheap, and lets heat from a UTH through without any significant loss

Paper towels/newspaper/butchers paper/ corrugated wrap are all great. I prefer corrugated wrap over all other substrates.