Structural role of the rear box sections?

[NoHachi]

The metal sheeting under the lip is what I meant by the tray. Its the bit making use of the high pressure area there and needs to be sturdy as hell if it wants to have a chance of pulling the entire car down to the ground. Thats why they made it metal and probably did indeed attach it to something solid like the x-member.
You can see from the pictures that the ae86s aren't running any ground effect (no skirts).

Other solutions to cleanup flow that I have seen include mounting the silencer in the trunk (if you have a silencer) etc. With the wheelwell removed and tank moved as well, you should have plenty of space to create a useable diffuser at the rear.