The gondola below is made of 5/8" pressure treated plywood.  I used treated lumber to insure that it would last a long time without worrying about rot.  I used hardwood for the ribs and top molding.  I found it cheaper to buy one large hardwood piece and rip it to size.  The drawings should be self explanatory.  I nailed and glued all joints.  I then used automotive body filler (bondo) for any visible defects before painting.  To protect the ends, I used 1-1/2" aluminum angle iron.   For the frame, I used the design from Tom Bee which uses a steel channel.  The trucks, couplers and brakes are also from Tom.

Below are pattern layouts to assist in cutting out the material.  As you can see, the patterns are for two gondolas.  Adjust as necessary.

Here's a few pictures during construction.

And this is the finished product.