I've done something similar in the past, with my Sony 505 (even used one of the types of journals you have pictured).
https://www.mobileread.com/forums/sho...978#post310978
Of course mine wasn't designed as a case to keep the reader in as I was actually reading. It was more of a protective shell. Worked very well though, kept my Sony safe & sound even though it was tossed in my backpack everyday. Ironically, a couple weeks after I started using a case I "paid" for, I dropped my Sony and broke it =( I always wondered if it would have survived the drop if it had still been in my homemade case.