I was lucky to find a gun display case a few years ago.  It's pine,
and it has a glass-panel door.  If you search for piano-roll cabinet
plans, you probably won't find much, but just do a web search for
"display case plans" or "gun cabinet plans" and you'll find a few.

I found one by Monte Burch that looks nice, and has a small storage
cabinet as the base.  You could store other various piano-related items
in there, like spare boxes, roll leaders, sheet music, etc.  The link
is: http://www.extremehowto.com/xh/printpage.asp?article_id=60483 

Before assembling the sides, just cut horizontal grooves in the sides
to hold slide-in shelving.  I use Masonite on mine, but it's sagging,
so I'd recommend 1/4" plywood shelves, with the side edges cut down to
fit in the side grooves, if necessary.  Make the cabinet depth equal to
piano-roll-box length, plus about an inch, just in case you need to add
shelf supports in the back for strength.

Bill Mackin
Hiawatha, Iowa