hi there,
I use the large paper portfolios you can buy at art and craft supply stores. They are $10-15 and I buy one a year. Works like a charm and not too bulky!

 

Clean/never used pizza boxes. I find that the pizza place is happy to give me one if I ask while I'm spending money there on pizza.

 

It seems you actually want to keep the physical papers themselves, so this may not work for you. But I read somewhere (maybe MDC?) that someone took digital photos of every single piece of artwork ever completed by her LOs and then had those digital photo frames all over the house with all their creations. That way if her LOs wanted to go find one of their pictures, they could go through the memory cards. It made throwing away the artwork less traumatic while also preserving the memory of it. Just a thought.

Pizza boxes are a fantastic idea!