Yakima/Thule roof box mounting question

I'm going to mount some permanent rack bars on top of my camper shell for carrying gear. I'd like to make it compatible to fit a roof box by Thule, Yakima, etc but I'm having trouble finding the measurement of the distance between the clamps that mount on these roof boxes. Does anyone know?

Also, anyone have any recommendations for steel rack bars? I'm mounting them to a fiberglass shell on my Tacoma. Was thinking of these AA Universal Racks off of Amazon. Anything else I should be aware of?

Those clamps on Thule and Yakima boxes are adjustable and can slide forward and back. I know for the Thule bars, if you go to the product page and download the product info PDF it will give you what you're looking for, I imagine Yakima does the same.

They are pretty adjustable the clamps slide in a slot. Manufactures have the specs on their website.

https://www.yakima.com/skybox-16-carbonite

"Minimum and maximum crossbar spreads: 24” – 36”"

For a rack I'd opt for a bar that can be wider then its towers so you can have the box really close to the side (easier to get stuff in and out) and still have room for a bike rack on the other side.

Ok thanks. Looking at the installation pdf seems like I have options. Anyone have any recommendations for rack bars?

I've been happy with my Rhino Rack Aero bars, but they can be pricey (I got mine off Craigslist for $120, including towers).

Those heavy duty steel bars seem like overkill for carrying a cargo box or bike (they're designed for carrying lumber, etc) but if the price is right then go for it.