I assume the stock holes are less than 1/4" ?
If so, a step drill would be the cleanest and easiest way to open them up to 1/4".
They are great for drilling clean holes in thin material.
If you do this, don't get a cheap one!
The cheap step bits are just that.
I have a cheap one and it is next to useless.
The quality one's are worth the coin.
I now have a couple of the Irwin brand single flute type. #1 and #4.
They are great!