The problem with the original post is the word "custom" without a definition of what you mean. It's almost a troll
Makers like Carvin have a few set basses with a ton of options, but you can't order 30 frets, blocks in maple boards or neck binding, for example. Tom Clement has a few set models with a ton of options, but he also has some things you can't order.
There is a big difference between 'full boat, full on, every single aspect Custom' and a very wide array of options for you to choose from to 'customize' your particular bass. Until the OP adds a bit of clarity these responses will either be helpful or not.
Also, fully custom will almost certainly mean a 'builder', like a Tom Clement, and not a 'brand' like noted in the original post.
Agreed- I own both a Clement and a Carvin and I love them both for different reasons.
My Carvin was my 1st foray into the world of "custom" and I love the bass. It was easy to order from their menus. But there are definite limitations on what you can order from body shape and pickups and preamp and so on and so on.
My Clement is my first "true" custom. Tom has a wide and ever expanding variety of body shapes. He will use pretty much any type of wood you want. The same for electronics and hardware, although his default stuff is really good- Hipshot tuners and bridges, and Bartolini or Nordstrand pickups.
Another plus to working with an individual luthier is the communication. Tom provides tons of progress pics as your instrument is being built. Plus he answers any questions and will work with you on any adjustments you might want. Those are the extras you dont get when you order from Carvin.
Once you order from Carvin, AFAIK, the next time you hear from them is when the bass is done and ready for shipping.
Again- I'm NOT knocking Carvin, I would not hesitate to order from them again-their build quality is outstanding.
But I'd much RATHER buy from a guy like Tom Clement.