My idea will only work for flat artwork, but so far it's all I've needed for my daughter's things.
With 3 sheets of poster board, stapler (& staples), and masking tape, you can make a portfolio!
Fold one sheet in half, with the fold going down the long side. Put the other 2 sheets together & the folded piece over the top of them. Leaving one half of the folding piece loose staple all around the other 3 edges of the full-size poster boards (you will also go through the 2 short sides of one half of the folded piece). The staples only need to be about 1/4" from the edge. Then use masking tape to tape the long side of the folded piece to the full-size piece, and also tape over the stapled edges (cover the staples). Voila! Portfolio :)
You will have one pocket that is full poster board size, and one pocket that is half-size, and then a flap to keep it closed. Your daughter can also decorate the outside of the portfolio!
I hope all that made sense. Send me a message if you have any questions!