I would check with different jewelers to get a unique ring design. Kay may not have the most affordable price for what you want or need. Also, the smaller diamonds tend to fall off, especially on pieces of everyday wear like rings
They've always had custom design but you want to make sure you ask for someone who is very experienced in it. It's a very long process if you decide to do custom. Custom is more expensive though. A lot of times you can find exactly what you want by 'tweaking' the rings they already have. You can always put in a different shape or size diamond or change the bands.
Im not sure if you can completely design your own ring. But i do know for sure that you can choose what kind of diamond and band match you want, also you can engrave anything you want into the band itself.