WHAT CAN USE BELOW MY WORKPIECE AS SUBSTRATE IN BLACKTOOTH LASER CUTTER?

A cutting surface. I bought myself a aluminum 'egg crate' mesh from my local HVAC company. They are used a cover for fluorescent lights and work great for low impact cutting surface. They come in 4'x8' sheets and are easily cut to fit. The other side is if you can find a steel honeycomb cutting bed to fit. These can be quite costly or fairly affordable depending where you are looking. The benefit of a steel honeycomb is you can use high power magnets to hold material down.