http://http://www.invertsupplies.co.uk/product-16.html
This will give you an idea of what it looks like. You can find it in pet supply stores. I suggested cork because it is mildew/mold resistant, and the humidity in your spider's enclosure is going to be high. Just get a piece that looks like half a tube, or even just a piece that sticks up enough to form a sort of cover over the burrow hole, but still leaves access to the hole. It looks like you have everything else well in hand; just place a water dish a short distance from the burrow entrance so it doesn't have to go hunting for it.

Good luck!