I think to be in the clear with the game warden you'd need to permanently affix the block of wood to the magazine follower so that it couldn't be readily converted back into a 20 round magazine (thus ruining the mag ... or at least the follower).

Even if you did that, you can expect to be constantly hassled by game wardens and you'll likely lose a day of hunting clearing it up (or worse the warden will think you're some sort of wiseguy and pull your license). Game wardens aren't known for their generosity, leniency or sense of humor.

Just buy a 5 round mag and be done with it, less headaches in the long run.

What kind of rifle you got? You might be able to find someone who could loan you a mag.