The bottom layer is more conductive, the top layer is less conductive, but still conductive.
Say, as a crude ballpark, you want between 1Mohm and 1GOhm from any point of the mat, to the ground plug. The only way to achieve this is a more conductive "spreader" layer, then a less conductive actual surface that prevents shorting things out with too little resistance.
Note that "dissipative" is just a fancy word for "a tiny little bit conductive".
For example, for the ESD mat I have, the top layer is conductive enough to be easily measured on a megaohm range of a multimeter.
AFAIK, there is no easy shortcut. Properly slightly conductive materials are not too easy to come by. Buying the proper ESD mat is likely easiest.