Riker display cases which are also called butterfly boxes.

I use the 12 x 16 x 3/4 but they come in many sizes and you can get them by the caese on Ebay for under $5 each.

They can hold between 10 and 25 plaques depending on the size and have a large glass display top and heavy cardboard edges. They are held together by 4 pins and can easily be opened to change the display.

They also stack very well and can easily be transported.

Hope that helps,
Doug