heater box/heater core?

1989 GMC K1500 5.7, i recently had a heater core go out on me, and when i was taking the cover off of the bottom of the heater box, i broke the plastic in half, so i went on with replacing the heater core, now im not sure how to reinstall the cover, it looks like part of the cover may slide between the upper section and the firewall? any help would be appreciated.

if you can post or send me a photo of the broken section I think I can help