Inside Evap/Heater core box - Need assistance on part identification

[SouthSeaPirate]

Decided to tackle this project to fix a leaky evap, and now most deff a leaky heater core.

So I have the box apart, flipping side to side letting all the junk fall out. When I move everything out of the way, I have a little plastic piece (see pic) sitting on the floor. Cannot figure out for the life of me where it goes.

 

[CharlesS]

Looks like a door to divert air flow....

 

[Hardkor]

IIRC it's a support for the heater core and keeps it in place. See if it is molded to the shape of one of the ends of the heater core.