Cracked plastic in engine compartment

I have a '94 Lesabre that I'd like to drive a big longer (maybe a year), but I've got a problem I've never had to address before in any car I've owned. It still runs just fine.

There is a large black plastic piece on the firewall, just to the passenger's side of the blower motor. I'd recently started smelling some engine compartment odors in the car, and I noticed a crack in this plastic. I thought I'd just patch the crack, but when I touched it, pieces of it just started cracking away. I've now got a ~6" x 6" square hole in it through which I can see some yellow insulation material.

I don't need a perfect fix, but I need to block this up somehow. I was thinking of just getting a couple of tubes of black silicone adhesive and spreading it on the insulation material with a putty knife, just to make an airtight seal that would last a while.

Any better ideas?

Check this thread:

http://www.automotiveforums.com/vbulletin/showthread.php?t=1139218