Thanks very much for those ideas. I have ordered stainless bars. My concern remains how stable they will be when I am smoothing the ganache, but I will soon find out. Someone mentioned taping them down if necessary. I will have to do something like taping when making a two-layer ganache.
I use a variety of techniques for slabbing
The first method I use is a 12 x 10 wood frame that is 1/2" high. This I made from maple with half-lap joints that are pegged. I lay the frame on a cutting board, lay a sheet of cling film on top, pour in my ganache, and smooth flat. When set I remove the frame which comes away clean, brush couverture on the surface, pick up the whole slab and flip over, remove the cling film, and brush couverture on the newly exposed side.
I also bought a set of frames from D+R in Montreal. These are S/S and quite nice, but expensive. However they come with a "movable wall", a bar of s/s that you can move within the frame to make any size you want. I lay a sheet of parchment on a large cutting board, then the frame, then fill. When cold, I run a torch along the frame to remove it.
I also have a set of s/s bars in 1/4" and 3/8" widths that I got made a metal shop for quite cheap. Very flexible and usefull. I toyed with the idea of cutting grooves in a cheap nylon cutting board so the bars can sit in and not move about, but that means I have to custom-cut a piece of parchment to fit in the bottom. I prefer the other methods
If you happen to have melted, tempered chocolate on hand, it makes a handy "glue" for the bars to the "table" and between the first set of bars and the 2nd set on top. You don't need much, and it cleans off rather easily with warm water and soap!