Padfolio, portfolio, envelope, whatever you want to call it, if you enjoy working in the field on loose sheets of paper, you’re going to have to protect those sheets before and after use.  I’ve made 2 of these fabric protectors for a size of loose sheet I’ve used in the past, 6″ x 11″.  That’s a size that uses all of a full sheet of 22″ x 30″ paper with no waste and the paper grain is aligned for easy thumbing through the pages.  I abandoned this project in favor of working in bound journals, but you never know when I might pick it up again.  Here’s the one I use for keeping the finished sheets together and safe on my bookshelves: 

Here’s another one in progress.  The inner support is finished and I’m auditioning fabrics.  This one is for 5″ x 7″ sheets: