Every Behmor 1600 Plus roaster already comes with a chaff tray inside. This is a replacement part.
Behmor put some thought into this tray that collects chaff and allows for great visibility into the machine.
There's a handle on the back that lowers the chaff-catching louvers. This allows the tray to slide out from under the drum. Be sure to clean out the chaff after every roast and occasionally clean it with a detergent that will remove any coffee oil build up.
Clean chaff out of tray thoroughly after every roast. Occasionally wash with hot water and mild detergent to remove any roast smoke or oil buildup.