Here's what Dave did.
1. Start with the 30"x30"x20" shelf (Ikea part 737.637.09). My 89"-tall unit has four identical shelves above the desk - adjust to your needs.
2. Flip the shelf over and remove the stiffener (shown below as the dark vertical line).
3. Mark off a diagonal line beginning 11-1/8 from the back of the sides of the shelf:
4. Carefully remove the metal support channels from the sides of the shelf where the cuts will be made to avoid damaging the tabs that hold them in place, or cracking the thin rabbeted edge of the shelf.
5. Make the cut and, using a router, plane, file, sandpaper, or cornering tool, round off or chamfer the fresh-cut edge.
6. Using the cut-off piece as a template, restore the notches to accommodate the 12" side rails:
7. Cut and file the edges of the metal support to match the rounded ends of the original equipment and re-attach them.
8. Drill and countersink a new screw hole about 3-1/2 inches from one end of the stiffener.
9. Trim about 2-1/4 inches from the same end of the stiffener and re-attach it to the bottom of the shelf.
10. Finish - or not - as you see fit, and enjoy!
(If you look at the picture of my finished project, you may notice that I didn't complete steps 8 and 9 - I just reattached the stiffeners and left it for later.)
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