I cut off a central part of the horizontal wall below the fans, extended a vertical wall with additional plastic, and added some details from scratch to imitate engine parts and the exhaust tube with connectors and bolts.
I sanded the three flat details on the inside of the back wall to 0.2 mm and cut out a bent lover part of it so that it may not obstruct new engine details.
Painted and weathered back wall. Only the upper part of the wall will be visible, everything below the shields-looking details will disappear into the darkness