I have the ESR back cover (10.00 USD) from Amazon - works well with the ASK. I also just ordered the Moko frosted cover too to show off my gold iPP. It should arrive today. I just really don't want to spend 70 bucks on an Apple case in two bland choices - gray or white....I've ordered a MoKo back cover - it's frosted plastic. Looks pretty good and according to the Amazon reviews will do the job admirably. At £6 I won't be expecting brilliant quality, but providing it protects it from the odd scratch and coming into contact with the gunk frequently found on desks at university I'll be happy.
Cheers,
Bobbie