First thing I'd do is replace the HEPA filter. A copper plate/foil may help, and it certainly won't disrupt the flow of heat, as copper is incredibly conductive (that's why they make electrical wires out of it), and poking holes would help with the airflow, but a new filter will probably make the most difference. I've never used a full-copper incubator, and the few serious contaminations I've had to deal with were related to the HEPA filter and poor TC practices.

If you haven't already, I'd institute a good routine cleaning program as well. Change the water bath each week (make sure you're adding fungicide to the water), and wipe down the whole incubator with bleach followed by an alcohol rinse each month. Make sure you clean all the bleach off, as it corrodes metal if left on very long. At the same time, clean and autoclave all of the removable components like shelves, rails, and water pans.

Good luck!

Copper for cell incubator to prevent contamination
In any version of Windows I have used so far (Xp, Vista, 7, 8) and all versions of the MSPY IME (RTM versions and the 2010 version)
There is this annoying bug that you cannot change or disable the hotkey for Chinese shape troggle (normal to double spaced chars).
The default is Shift+Space, and cannot be changed from the language input settings pane in the control panel.
Here are a couple screenshots to show the problem:



After that press Change Key Sequence

This dialog will appear:



Disabling it, has no effect, i.e. Windows will ignore the setting.
You will notice I already managed to change the sequence to Shift-None by using the registry, but Windows still uses Shift+Space for shape toggle, which is really annoying when you type Chinese faster (especially if you want to switch between English and Chinese).

Now after you changed or deleted the key with the dialogue, the setting will not be persistent with hitting Apply. Changing the corresponding registry value and rebooting also doesn't help, as the following screenshot demonstrates:



I hope anyone has experience with this problem.
