you can also drill one hole on each end of the oblong and then proceed to drill a series of holes between them by overlapping the drill holes ,that when you are done it will look like waves on both sides of the oblong ,then take a hand file and file them down make sure you had set up guide lines ahead of time so you dont go too far with the filing ,which means set up
2 lines for the width 2 lines on the end for the length and 1 one line right down the center for your drill guide . It's best to use a drill press if you got one for this method ,but you can use a hand drill but be extra careful of catching of the bit when doing that and always make sure your piece is secured ,it takes awhile to do but you can get it done by yourself and without expensive equipment . If in a hurry and dont have a milling machine take it to a machine shop to do and explain and if possible draw a picture of what you want . good luck