IPad/Keynote Self-guided presentation

Setting up a self-guided tour of a non-profit car museum using Keynote on IPads that will be lent to visitors who prefer a self-guided tour instead of a human-guided docent tour. Everything looks promising with a hyperlink-only presentation that is shaping up. (I am disapointed in a couple of missing features in IOS Keynote that are present in Keybote 09 however: ability to add voice-over narration or sound tracks to separate pages, and the ability to do "smart-builds" for multiple image slide shows. The first can be done in Keynote 09 and import to IOS but the second won't even import.)

I have found that a links-only driven presentation in Keynote is similar enough to web browsers that it is intuitive enough even for non-tech-savy users. But one problem has cropped up. People unfamiliar with Ipads or Iphones will push the hard button on the bottom of the screen and exit Keynote to the home screen or they will put the IPad to sleep with the small button on top. In either case they are then completely lost.

Does anyone know of a case for Ipads that covers the mechanical buttons so they can't be accidentally pushed?