On the right track but Id do something different on the rear. That strap /buckle may abrade a tire sidewall or buckle will scrape up a rear rim.

When I was a college student and poor, I made a wood DIY rack of different design. Im glad I did cause I drove into a garage and splintering wood of the rack saved my frame and fork from damage. When I got the funds, I opted for a thule rack. I felt better at 70 mph knowing that I wasnt going to kill someone! Im not making a comment on your design, just the liability of a homebrew design.

I just now started carrying kayaks with a store bought vertical stacker after years of carrying bikes without thinking about it. I really get nervous with a 15ft boat up there and cross winds. I guess its a comfort thing... I drive slower in cross winds but it still makes me nervous. I couldnt imagine having a homebrew kayak stacker design up there even though conceptually its a simple design.