Browsing around an Office Max store she spotted these foam boards (picture above shows them at Walmart). Lightweight, and cheap! It's a thick foam core board with a thin layer of cork. We bought some wood lattice-like strips (like a yardstick size/thickness) at Lowes and hot-glued them to the backs at the top, middle and bottom of each board. Then I needed just one screw at the top and bottom of each, attaching through the wood strip for stability.
We will add a 24x30" raised section to the table at the left end, on Ikea Capita legs. You can barely see them on the right in the second picture, raising up a different top. This will give her a standing-height work surface on half of the large top. ( In our experience you do not want to use the Ikea adjustable legs, they are just too wobbly.) If you add a second top area on Capita legs you wind up with all the space under it to put more of your stuff that you never manage to put away.
Here's this guy's answer to the more expensive flat-file cabinets: