I thought there would be an assembly service considering the current v2 gb has this already.
In the past, I have seen where a 3rd party will do an assembly service as a separate group buy, but it doesn't always turn out well. My suggestion is to have your order ship directly to the person that you want to build it.
For example, place an order with TJ, or any other assembler
listed here and then have your order shipped directly to them. Once they receive it, they'll build it to your exact specifications and then ship it, completed, directly to you.
Based on my past experience with assembly services, working directly with an assembler directly is the best method.