is this the lid you're talking about?...
you could certainly go to a local ace hardware and have them cut a piece of glass 19 1/4 X 9 3/8 and that will sit perfectly on top. use a rasp or dremel to smooth the edges. then you just use a razor knife to cut the lip that sits in the groove off the lid and it works just fine. for better details, check out the pics in the fun with humidity thread.
also... if that's the hood you're using you could put a fan directly in it if you shaved it down right. with the glass there you wouldn't have to worry about losing humidity to deal with heat either.