I'm probably not qualified to answer this question but I think the best mat covering for a bin is harvested vermicompost. If there is none of that available then sifted out bits are perfect for a top coating to a bin. This helps inoculate the new additions and assures a nice area for worms to be near by the second the new additons are ready. Nothing is lost by putting harvest material through the bin twice because it is soon rehavested and not lost. Just a bit of time.

In my case the egg shells actually get to be used as intended instead of passing through the bin in a newspaper shelter.

A major reason for a lid is to decrease fruit fly area and stop sun from drying and worms just like to live underneath a sheet of wet cardboard that is set on top of a bin. The air moisture ration is perfect there.

andy, the deeper your bin or tiers are, the more compaction you will see over time. If you don't have one, I'd suggest you invest in one of those hand rakes and give your media a stir once in a while. Red wigglers in indoor bins are not as fussy about being disturbed as some other composting worms. If you tiers are smelling anaerobic you might cut back on the feed as well. A fermented smell isn't that bad, so be sure you know the difference.

Don't have a FT tray system myself but I'd definately second the 'layer of harvested VC on top'. Just enough to cover it lightly. Seeds the feedings with bacteria, keeps things moist, cuts down on any smell.

I've heard those stackable systems are not the most breathable, so anything that gets you more should be good.
I've always wondered if making a hole in the center of the VC in the lower tray levels would help. Then air could pass from top to bottom freely in the center. Just a thought, never done it or heard it being done.

Also if you're got food types that tend to turn into mush, you can always add cardboard with it. Cardboard more than paper for aeration as it will hold it's shape and make air pathways more than paper will which can clump just as much as the food.

"I've always wondered if making a hole in the center of the VC in the lower tray levels would help. Then air could pass from top to bottom freely in the center. Just a thought, never done it or heard it being done." Great idea. I do not have a tray system but I have heard of people leaving the top or trays ajar to let in air. Newer systems seem to incorporate that in. Also maybe just bedding in the bottom and very top tray. It will get moist a bit on its own.

outdoor andy if your system is going anaerobic maybe add a ton more bedding. Maybe even a dry block of this coir everyone is talking about.

We all want to feed our worms to nurture them. Maybe think of bedding as fluffy pillows for the little guys and gals.

Great advice from everyone. Thanks for the responses.
--Andy

Two of my bins have old towels (rag bin is stuffed) that the worms enjoy lolling around in. On top of the towels is a semi-permeable inorganic polyurethane-type of sheet, that is cut to be a bit less than the size of the rubbermaid bin, and that seems to keep moisture in, and allow for a bit of air from the airholes that circle just below the rim of the bin. Followed by the lid. Everyone seems to be happy with that.

It took about 2 months for the worms to eat up an old cotton tea towel. So now I'm curious how long the new "old" towel will last (an area just above where I've been feeding them is gone completely, so somebody likes it!)