Do you have a basket in the SFB? That's a pretty simple and effective mod. I have a grilling basket from Lowes in mine. Cut off the handle, hang it by a couple of bolts. It get the fuel off the bottom of the box for better air flow. That helped mine, along with a different baffle/tuning plate ) two actually. I got lucky and a guy I knew who is a welder made mine out of some heavy steel. Has holes in the plates, and a bend where it meets the SFB to deflect the heat. I think that works better than the upside down tray.
If you do a search here you will find some great posts with pics. Keep working at it, you will get it. One more thing -- use good thermometers, don't trust the one that came with the unit. Get a good one, calibrate it, and have it at grate level.
Good luck -- let us know how it goes.