brokenhead -
Are you talking about a cover that covers the heater motor? If so, I assume that you replaced the original motor with one from an auto parts store.
<CLICK> to see the original style heater motor through the firewall. (This motor is actually a 12v 1960 Chevy heater motor.)
There is no cover over the motor. Some 'Bolters installed the aftermarket heater motor inside the case of an original heater motor. Some aftermarket motors are small enough to do so.
A rubber gasket seals the gap between the motor and the firewall. A metal ring secures the gasket to the firewall.
Jim Carter sells the metal ring ... "Heater Cowl Round Firewall Ring 1947-1955 4 5/8 Appox" Part #MEH254
I don't know a source for the rubber gasket. You can cut one from approx 3/16 rubber sheet. Cut the center hole to fit snug and stretched around the motor. Trim outside to match the metal ring.
<CLICK> to see the motor and the original rubber ring at the left.
<CLICK> to see the other side of the rubber ring.
Now then - if I was totally off base with my answer, then Jim Carter also sells the block out plate for firewall that covers the hole for the heater motor.
"Heater Motor Hole Cover Plate 1947-1955" Part #MEH253
- Lonnie