I'm a huge fan of digital over paper but what would you want on the custom stationary. A typical paper Zettle has:

A unique identifier line or box

A content section (I'd assume that can be most of the front and all the back

A related notes section.

I'd think a typical 5x7 index card with (3) in the top area, (1) in the lower left and (2) on all the rest does the trick. The main place I could see stationary helping is if you want the identifier to have distinguished sections. For example lots of people are using the Dewey Decimal System or Britanica Propedia classification for simplicity ... while I think Library of Congress classification makes more sense since it is available and agreed by the publisher. You could potentially use both in the ID section.