Doug Fattic 

You can buy the right one from MSC for $37.25. MSC Part # 08862161. Another option is MSC Part # 45403334 for $44.34. Then you can loan yours to other amateur builders. You will never miss or regret spending the money. If however you just drill it out with a 5/16" or 8mm bit only without countersinking, it will bother you always. I speak with confidence about your future mental state because my college major was in psychology and my graduate studies in counseling (so I could be a high school counselor). When I was apprenticing in England, I got guff for downgrading my social standing by learning a manual skill. I couldn't have cared less about their opinion just like your joy of making your frame look just like you want it will allow you to ignore all those that think you could have spent your money more wisely (on what I ask?) by just drilling but not countersinking your fork.

MSC is a bit of a nuisance to order from. You have to establish an account first. Neither are they the cheapest to buy from. One of their biggest warehouses is local to me so I order from them all the time. They have those annoying promotion offers because usually it requires a $ minimum.

I should add that you can use a 13/32" end mill. However at a minimum the fork needs to be held securely in a drill press but more preferably in a vertical mill. And you somehow have to make sure the bit is in dead center and the fork won't/can't move. Using a drill bit is a bad idea because the bit will flex under load.