I knew there had to be a simple and cheap way to protect a HHKB during transport, and I think I've finally cracked it. There are neoprene wraps for wrapping around cameras, lenses, etc. I bought one sized around 50cm square.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=130312044752As I took it out of the mailing envelope, I noticed it was already folded to nicely match the width of the HHKB. It could be used as a very thick wrist rest in this form:
To cut down the height you could unroll it a bit and put some under the keyboard:
There are many ways to fold it, or use it fully unwrapped as a mat/wrist rest.
Or you can simply use it for its intended purpose, protecting your valuable item. There are Velcro-type strips to secure it:
Wrapped as above most of the layers are above the key caps, giving these most fragile parts the most protection.
It may not be as convenient or classy as the
Waterfield Sleeve, but it's easier to get, cheaper, and more versatile. And guaranteed to fit! Anyway, for only £5 delivered there's no excuse for leaving your HHKB unprotected when you take it out on the road.