In reply to JohnV:
If you have a router or table saw then mark it up, make sure you won't hit screws and it'll still be strong enough then knock as much off the corners as you can with a chamfer bit or 45deg cuts before finishing with a plane. Planing ply is a pain.
Alternatively round over both edges with the biggest radius router bit you have and live with the imperfection, I think my cheepo 1/2" set has a 3/4" radius round cutter. My big router would swing a bigger cutter but I wouldn't much fancy being near it!
One final option if you have the router in a big table (as I do with mine) and a willing helper is pivot a couple of wooden blocks on screws driven into the main axis of your desired round. Add a fence to the table to aid positioning then stand the whole thing up on edge with your helper so the pivot blocks rest on the table surface against the fence. Set a big end cutting bit up to the right height then rock the board back and forth through 180deg over the bit sliding it slowly along the fence to perfectly nibble away the desired radius. Not worth the effort!
jk