Step 1: Measure Your Bookshelf, and Procure Your Materials.
Our bookshelf is 31 x 15 1/2 inches. So we procured some woods with which to build a tray that fits on it.
Our base is made of 1/2 think pine board, cut to a 32" x 16" rectangle - a wee bit bigger than the top of our bookshelf, so as to make for a not-too-tight fit.
The walls of our tray are made from 1/2" by 6" pine planks. Two six foot long pieces were more than enough.
We used some handy dandy corner braces that we got from the Home Despot (see pic), along with some wood glue and some longer skinny wood screws suitable for screwing the planks in at the corners.
Even though we are prejudiced against hippies, we still act like them sometimes. So we use natural untreated woods.
Step 2: Cut and Sand the Wood
If your wife is pregnant, consider exploiting her nesting instinct energy, and make her do all the sanding.
Step 3: Prop the Floor of the Tray Up
So before you attach the sides to the bottom of the tray, put some scrap would below it, so it sits about 1 1/2" above your work table.
Step 4: Glue and Screw on the Sides
Now screw the handy dandy corner brace to hold it all together. If your floorboard is splitty, consider drilling a small hole first so the screws don't make it split.
Now repeat with the other two side planks.
Step 5: A Bit More Screwing
Drill holes in the side walls, and put two longish (~2 inches or so) screws in each corner.
You are almost done!
Step 6: Put Something on the Bottom to Keep From Scratching the Bookcase
As you can see, I also put an extra bead of wood glue in the seam between the floor and the walls. It can't hurt.