I was lucky enough to do most of my previous laser work on a Universal system that included a precise and motorized z-axis. Here is my K40 solution: A Stainless Steel Lab Jack Scissor Stand. I found it on Amazon for $17.99. It is simply a miniature scissor lift.
Since the K40 laser I believe comes with a 2" focal point lens, you need to adjust the depth for varying thicknesses. So for example. A 1" thick piece will need to be 1/2" deeper in the unit than a 1/2" piece. I'll need to make a precisely 2" measuring block (if the focus is indeed exactly 2", I'll need to test) to sit on top of the work piece, and be able to adjust it from there.
I have of course removed the goofy spring-loaded clamp that is standard on the K40and placed the scissor stand in the void. In calibrating the cutting area of the machine, I have temporarily taped down a small carpenters square to find the largest working area and a way to align work pieces each time. I suspect I'll be cutting a larger opening into the aluminum base inside the laser a bit larger, probably just under the 8" x 12" maximum. Since most of the material I cut for my artwork is 1/8" thick. Still, I want to have the system set up so I can always quickly drop a piece of 1/8" thick stock into the unit without having to adjust the z-height. So the scissor stand is mostly for the odd non 1/8" thick material that I'll want to cut or engrave.