Otherwise there is The Alternative Reef (The Alternative Reef) who does some custom work. You will have to contact them to see about the structure though.
Aqua tecture is their store front. Be ready to be patient. I have a friend who is gong on 6 months waiting for his. Communication is sparse. The good news is you pay nothing until they ship so there is no risk. I used Greg at cultivated reef to get mine. Took me about 4 months. Worth every penny and worth the wait.
I bought all my rock from them had a custom made piece. It took about a month longer then expected. It was packed well but came cracked in half. They offered a refund or showed me how to fix it. I choose to fix it and they gave me a couple frag rocks for the trouble. I kinda wish I paid more for a bigger piece. But everyone that sees the rock in person loves the shape. Look for my build thread I have pics of the rock. I would still recommend them and thinking about switching out a frag rock for a bigger structure.
I have bought two of the centerpieces in the past that are currently in my 120. I didn't have them custom made, they were already in stock, but for those the shipping time was fast and they arrived in perfect shape.