Duct Accessories - How to add inline without breaking the host

Can you create a duct accessory family that will attach to a duct (therefore picking up the elevation and size of the duct) but not break the duct into 2?

With Break Into you end up with 2 ducts

With Attaches to it goes all wierd and starts to attach to the side of the duct.

I want to be able to place an accessory onto the center line of a duct and have it surround the duct without breaking the duct... which settings should I have in the family?

Thanks,