i couldn't help but notice the nice large(ish) four holes going through the PLCC. You can see the chip it goes over through them. i wonder if there would be enough air leakage around the plcc pins and Lisa for air to get sucked up and out through those 4 holes. placing a fan on the pcb over the holes and sealing around it with hot glue to make sure any air pulled out has come across the chip instead of just being bounced/buffeted around the cavity from the top.
a more risky and destructive method would be to remove the board and plcc plastic between the 4 holes and stick a heatsink on Lisa