I don't have any ideas, but I think yours is great. My DS has always been in a playpen (after leaving our bed at 1) and I had a really hard time finding sheets. The only place I could was ToysRus.
As a user, my issue is the leak-through. I can't clean the playpen mattress so its getting a little funky. It is also a little bit hard (board with cloth covered foam) so something a little squishy would probably be nice. The sheets I do have are flannel fitted with elastic at the ends which is really useful. If it had some kind of fleecy or wool (recylced wool blankets?) under the sheet, that would be awsome. I'm not crazy about plastic (waterproofing) in baby's bed, but maybe there are other waterproofing/water resistant options?
Don't know if I can ask here, but when you're up and going, PM me and I would probably buy one.