When you get right down to it, a convection oven is just an oven with a fan.
It might blow the air over the heating element, or in a special pattern to maximize even heating, but its still just a fan.
If you are using a junker oven anyway, it should not be too hard to rig a metal blade fan in it.
Drill a small hole, mount the fan inside on a long shaft with the motor back away from the side of the oven.
If you could find a couple of aluminum fan blades to fit that shaft also, you could bend the blades flat, clamp them on the shaft between the oven and the motor, and they would wick the heat away before it gets to the motor.
The fan blade inside could be aluminum or steel, just not plastic............