I am planning to make a shim to treat from above. In past treatments, some frames have been charred on the bottom. The shim I am thinking about will be about 3 inches high. I am planning to use some metal mesh (as wide as the tray) to rest the tray on and keep it off the frames. The mesh will extend all the way across the box and secured to minimize sagging. I am thinking about putting the opening about 1 inch from the bottom of the shim to help keep off the frames.
Any ideas or suggestions?