This is a thruster designed for the DVH Props rocket pack.
It's meant to install via a 1/4" hex head bolt, using the thruster-bolt.stl part. Glue the bolt into that part, then insert that part into the side of the thruster. It will both make the thruster arm connection much more stable and allow you to screw the thrusters on.
The base STL is just a little detail piece to make the inside of the thruster look less like a hollow cone. Not based on any reference, just something I threw together. If you use it, just glue it to the inside of the thruster... it will sit maybe 5/8" in or so from the edge.