I have previously bought a personal sized pencil board from TarokoShop on Etsy. It is nice and durable plus the grid helps no end as I often need to use graphs.
When I did an Etsy search for an A5 plastic traveler’s notebook pouch (not that easy to find) and TarokoShop popped up, I bought a couple immediately. It is so nice that inserts and accessories for unusual size covers (A6, A5, and personal size) are popping up more often these days. A year ago you could get regular size and passport size TN accessories and inserts, with Field Notes and others being as scarce as hen’s teeth. I am glad to see that some sellers (like Monique Vanmeulebrouk) are even offering B sizes and A4 in covers nowadays.
When I was perusing the other items on sale, I saw this beauty at a very good price. I adored the pebbly leather in this example. I am such a sucker for pebbly leather!