I bought this hard case:
eBay - The UK's Online Marketplace
Internal dimensions now that I have it in my hand are:
Case: 26.8cm x 19.1cm x 3-4cm
Prime: 26.3cm x 18.1cm x 2cm
As you can see, it's a pretty close fit in theory. Have to wait 5 days before I get my Prime to confirm it's a good fit.