(and of course you may)
The original design did actually have a completely round hole.
I was worried that it may limit the kind of necklace/chain you use so squared it out a little to increase the size.
Depending on how you attach your chain/cord, it can still hang straight (over and under similar to the 2nd photo in post 1), though a smaller circle is doable.
I tried a more oval shape but didn't look as nice.
possibly a very slight angle towards the center at the top side could work too.