Looking for a bulkhead - PA-28

Airtex carries them, something like $130 or so. Pick the fabric
you like, give them the serial number, week later you get a
really thing but big box. They're amusing to install. Get your
local A&P or builder to help you fasten it.

Note exactly where the existing holes in the airframe are located.
In fact, make a paper pattern and check it at least 3 times.
Then drill into the bulkhead following the pattern.

If you're *really* lucky and did it right, the screws will go
right into the airframe where it forms the frame for the bulkhead.

There are other options but this may be the simplest.