[Richard]

Yep, I crawled in the plenum. And yes it was wet and nasty.

One other thing that I did was to take the plastic grid from a cut to size filter obtained at Lowes, and cut the grid to fit exactly into the slots under the steps. The grid was 1 by 1 inch plastic and easy to cut with wire nips. The grid provides great support for the blue disposable filter media that I cut to fit. Now I don't worry about the filter media getting sucked into the plenum, or other objects such as coffee filters that will block the air flow.

The biggest challenge with the SCS units is making sure the air flow is as unrestricted as possible.