First question here, but its been bugging me the past week. I've been looking into building a Hard shell Roof Top Tent (RTT) and I've seen a few people building them. I want to try and make my own, but most use 80/20 Aluminum extrusions, but I want to try and keep costs down, so I thought about rectangle aluminum tubing. Aluminum tubing for the "frame", with a MaxMetal or similar for top and bottom, and use riv-nutz to hold them in place. Buuutt, from what I've seen with deflection, aluminum tubing might not be the best choice. I want to keep it as lightweight, and rust free as possible, while being able to withstand high wind speeds (On top of a car), and be strong enough to last for at least 4-5 years. I was using some deflection calculators, but mostly all have it where its supported at one or both ends, the RTT will be supported about 1/4-1/3 inwards from the ends, and most weight will be in the middle/towards the ends, but over the cross bars (Person sleeping in it). I want to keep a similar look/profile as a Go Fast Camper (GFC RTT) or a Terrapod RTT.
Dimensions wise I was looking at either of the combinations (What I had read, the top portion isn't very structural, but the bottom half is, hence bigger material)

1" x 4" Bottom, 1" x 2" Top
1.5" x 3" x 0.125" Aluminum 6061/6063 for both top and bottom
1.75" x 4" x 0.125" Aluminum Bottom, 1.75" x 1-2"(?) Top tube

Overall outside dimensions I want to stick with - OD 84" x 48" x Maximum 6.5" tall (For best MPG's and minimal added height, only for one person, 2 if their friendly)

I'm assuming the MaxMetal (Thinking 3mm on top, 6mm on bottom) will help strengthen it a good amount, using stainless steel 10/32 Riv Nuts and screws to secure it to the aluminum tubing. I want to keep it lightweight, preferably under 115-120 LBS finished. So questions that I have are -

Will the aluminum be strong enough for the frames? Will the MaxMetal stiffen it enough?
Would using Steel rectangle tubing be easier, stronger, lighter? (I want to stick with aluminum if I can, so I don't have to worry about rust)
Which would be the best to use? I think 1" might be too small for the weight it would need to handle, so maybe the 1.5" or 1.75"? But then the MaxMetal might stiffen it up enough to use the 1"?

Thanks in advance to all that can help.

(Link that I saw, but they had used 80/20: