I thought diploma covers was an interesting idea too. So, I looked that up and here's a few links. Getting it embossed is an interesting idea too::
http://trophycentral.com/diplomacovers.html
http://www.schoolunif.com/paddipcov.html
http://www.johngaltbindery.com/__Ins...ma_covers.html
http://www.academylooseleaf.com/cert...ma-covers.html
Some of them have a moire lining.
I'm sure there's many more. More pricey than the boxes, but still not bad.