Jennifer,

The easiest thing to use with control is a file.
Use the plywood as a guide filing until you are flat on the wood, then to to the other side and do the remaining lump on the other plank.
A little practice and you will be able to remove the glue but virtually no wood.
Take your time and do not use a rasp.

Keep a brush handy to clean the file and keep it cutting smoothly.
Another possibility which, like the file, is flat and will cut the high spots well, is a stick with a piece of sandpaper stuck to it. 80 grit adhesive paper (comes in a roll) is my favourite but you can glue ordinary paper to a stick, or use a PSA sanding disc cut into strips. These are 'throwaway' tools.
To protect adjacent surfaces, a piece of sheet metal (aluminum flashing scrap) held in your other hand or taped in place can help prevent accidents. That's a trick from the guitar repair world.
I also wouldn't use a rasp- but because I don't want to damage my rasp with epoxy/filler!