Having just read Stuart's ScotGrid blog post about cooling in the top of racks I thought I'd let you know about the panels we use.
We have been specifying that all empty racks slots should be filled by blanking panels since our 2007 purchase. The they used to use metal blanking panels.
These days they tend to supply the 1U APC plastic clip in panels, as can be seen in the RH rack in the photo.
These cost £25-£30 per pack of 10 but we managed to get a bulk (200) purchase in 2008 which worked out at about £1.69 each.
http://www.apc.com/resource/include/techspec_index.cfm?base_sku=AR8136BLK200
