Will try to post a few pics of what has been working very well for me but I wanted to get this description posted today. Previously using a modified bread machine with a hand held heat gun to make my roast batches using a drilled hole for a heat probe to monitor bean bed temps. I liked this better than an infrared pistol as I thought it gave more accurate readings and one less thing to hold. Setup produced fairly small batches (under 400g were the norm) and I wanted something better. I noticed a neighbor had discarded a soymilk making machine and thought it might have some useful parts. After taking it apart I was able to repurpose the plastic body as a bean cooler by adding a fan to the bottom that sucks air out and adding two small strips of wood to act as legs. A stainless mesh screen inside and a switch completed that project. The stainless inner drum was perfect for my roaster body being slightly larger and stronger than the thin aluminum pans found in bread machines. I mounted this inside a stainless mixing bowl that was a bit larger and filled the empty space around it with vermiculite and sealed the top with plaster (will improve this part soon). I needed something that would stir at the right speed and found someone selling a slightly used laboratory stirring tower setup online. I modified the bottom of the stirring rod with a wire whisk and gave it all a test. (The lab stirrer had an extra arm that I used to mount a hair dryer holder and that freed up an extra hand.) Using an electronic temp control heatgun I set it at 390C and my setup worked like a charm.
600-800g batches are about what I produce but more is possible. The bigger drum had more mass for heat retention and I was able to insulate around it with the vermiculite. Once second crack was done I had to lift the stirring tower away (a bit awkward but working on this issue) and then using hot pad mitts lifted the stainless bowl and poured the beans into the cooling pitcher. A few mins afterward you can tell when cooling is done by hand testing the exhaust air temp. Not warm anymore? Beans are done! Grind, brew and enjoy!