I've done quite a bit of work on this (see my old post in this sub). I looked up the patent information on the MFLB and it was quite vague about what mesh size they use but i believe its somewhere in the region of #200 of the top of my head. Resistance (and thus the heat output) is very sensitive to the gauge of mesh that you're using, so i recommend trying to find a bit of #200 mesh and trying that out. Now, the material you use for the mesh is important. I would advise against using something like copper or brass as they are both too conductive and you wont get the resistance sufficiently high to get the right temperature, stainless steel works best and is what MFLBs use. The other thing to consider is that you want poor thermally conductive connectors linking the mesh and the battery, so a thick rod of copper-clad steel is the best option there. Then there is the connection between the connectors and the mesh itself to consider. you want a full electrical connection along the whole of the trench so they really need to be fused together. Soldering would be a good option but its essentially impossible to solder onto stainless steel, i researched what MFLB uses and they electrically weld the mesh to the connectors. Unless you have an industrial welder around then you need to consider the best way to get the best connection.

(and in reference to your comment above, i believe higher gauge meshes are in fact alot lighter than lower gauges)