I don't know if such a product exists, but you could find a case that prevents the home button from being pressed. My kid has a toy case which holds an iPhone that works this way. She can use whatever app is open, but a hard plastic strip in front of the home button prevents her from exiting the app (and she can't open the case because she's a baby).
You can use parental controls to block a lot of the apps on the iPad so that it's very limited. And then you could add website bookmarks to the desktop so that it's easy to launch these sites you want people to read and use. And then move all the remaining icons into a folder so that only your bookmarks stand out. People could still do web surfing, but with so much locked down, and it being obvious and easy how to launch the content you want them using, you'd probably still get the results you want.
Or you could save some money and just print the URL to this info on your wall along with the password to your guest wifi. Your patients can use their own iPhones/iPads to take your survey and appreciate the wifi without the chance of them breaking your iPads.