Your best bet is to make a flap that contacts the housing in a manner that it can deflect with expansion from heat. like this:
If you give the sides a bend they will flex but still make firm contact with the housing and the expansion should be minimal enough that they will keep there shape and maintain contact.
The innermost edge that seals against the back wall will have the strength of the flange bolted to the housing to keep that in check.
If there are any leaks, they should get crudded up with carbon and seal up for the most part after a little while.
That would be my plan if i had one and had this issue.