That has "ears" in order to screw the grill on the down side of the dropped ceiling panel to it. A contractor that worked on another part of my basement installed one but I think he gets his parts from a HVAC supply place, the typical big box stores don't have them. Anyone know where I can get them?
i used to see a lot of hvac guys screw 1x2 to the boot to hold it and also have something to screw the register to. you could also bend a flange on the boot yourself. i have seen them screw the metal angle they use for returns on a boot also. if you absolutely need one try a heating and cooling supply
I go to a local heating supply company that has their own shop in house. They bend up anything I want. They usually supply separate L-shaped tabs (ears) to fasten to the ends of the boot.