Depending on how much weight you are looking to carry with the rack Yakima and Thule both make mounts that will fit into a factory rack with towers. What is the vehicle you are putting them on? There are some creative options using 80/20 aluminum that give you a very modular setup.
I use Thule shells.
They are dual side opening (keyed).
I wouldn't use the single lock/single side opening cargo shells. The convenience of both sides opening is worth the $600+ .
Necessity is a mother! I was able to buy a few used parts (we have a great source for recycled building materials here in Hood River) and craft these rod holders. This is just a prototype, but I think it will prove the concept.