You're very welcome, it's not a problem at all answering these questions.
You can do some research here:
www.etrailer.com
I've been buying from them for years, never had issues with them (luckily I never had to return anything).
I believe the MAX LOAD RATING is written on the OEM roof rack. Go check out this website:
www.mazdaparts.org
I've not had any issue(s) with MazdaParts.org (Med Center Mazda of Alabama) neither.
I bought mine, a Yakima RocketBox Pro 11 from eTrailer because I needed a smaller box than the Thule Evolution cargo box that I already have; and also, I just needed something that I can carry skis and poles (we're a family of 5.. with Mazda5!).
I only put it to one side because I was planning to mount a snowboard attachment (in case I got into that too) later. But for now, not being used.
If you get a bigger box, you can put it in the middle of the roof rack. In fact, that's how mfg. will most likely recommend you do.
You can move the box from/to another car, most of them have universal bracket that will fit the factory roof rail, round, and square bars. I like my Yakima because it has no separate bracket that I need to attach to the box; it's all self contained. All I have to do is to twist the knobs inside of box to tighten the clamps onto the bars.
Best of all, Yakima is running a rebate right now until mid March on their cargo boxes; but only if you buy from authorized dealers such as eTrailer. I don't think all sellers on Amazon are authorized. I got $75 rebate on mine.
Yes, you can use the steps on the Mazda5 to access it; though it would be nice if I can add an aftermarket side step just for that purpose.
It will generate some wind noise without a doubt. Smoother the box, the less noise, and less taxing on fuel economy. I think Thule is most aerodynamic with smooth surface; but of course, that will cost you a bit more than Yakima.
I don't know if you can get a copper red one, I'm sure you can get it painted but not sure if that would be ideal. I think for your car, black should match fine.
Why not take advantage of Yakima box with rebate, I highly recommend it, and it's a breeze to install and operate.
Thank you for taking the time. The roof/cargo is what works for me. Security not an issue, we load at our house, unload at vacation house. Weather not an issue. Is there a box that I can find which will be OK for the roof rack max load, and be more even (i.e. yours looks like its on one side), or do I need two boxes to cover most of roof of the car?
Lastly..are the roof boxes changeable to other cars (I have a Pathfinder with roof rack..so would mazda 5 box work on that pathfinder roof rack or car specific?)
Also instead of side step, can you step on the floor of the vehicle and access roof box?
Also is roof box really noisy and effect MPG a lot (not abig deal?)
Lastly can I get a Copper Red matching roof rack?