The plastic cover on the firewall side of my '95 heater box disintegrated too. (Here I am talking about the large air chamber that is attached to the passenger side of the firewall and into which the blower motor is bolted.) Pic below shows nice "new" JY cover in place.
When I had my engine out, I went to the JY and got a good one and replaced it. It was not that easy a job with the engine out! I can't imagine it could be done at all with the engine in place. The motor cover I purchased from the dealer. IIRC I paid about $10 for a new one. That, as well as the motor, can easily be replaced with the engine in the car. You do want to make sure the little tube fits well - it provides cooling for the blower motor. If it leaks or sucks in hot engine air, the blower motor will not last long.