Skylines at Moorabbin has a range of checklist/note books for quite a few different Aircraft models. I'm pretty sure I saw a PA31 amoungst the lot.
Nicely bound in a ring binder format about 5 to 6 inches tall with a nice picture of the Aircraft on the front.
Here is the link to their page for the PA31
Piper Chieftan PA-31-350 | Skylines pilot shop