The pattern around the cut out looks good; they're close enough together that it will sparkle nicely (smaller stones with AB coating can work and give you a good shimmer). If you want a bit more density cheap, then, get some ss10s to fill in here and there - the irregularity works well with AB as they refract light differently.
However, I wouldn't bother with the purple crystals at all; they won't show. I'd use AB crystals instead for the body. And, you need to go way bigger if you're just doing individual crystals; I'd not go below an SS30 myself.
And, if you want a twinkly effect, where you have the illusion ... get some tiny ones (honestly, I use ss5 or 10) and scatter them over the illusion. You could see the twinkly effect from the nose-bleed seats in our arena ... and it was cheap!