Jack,

My grandfather built me a bed years ago just like what you are
looking for. It was all made from plywood. Think of it as a super
short dresser with only one drawer.

When the drawer was closed, it was just an average twin bed. When
the drawer was pulled out, that effectively made it a double bed.

Only difference is the drawer that pulled out had a top ( hinged )
It would open up like somewhat like a diekens( sp ? ) bench and inside
it was storage for the foam ( for the pull out bed part ) and spare
sheets and pillows.

Hope it gives you some idea's

Todd