Id stay with aluminum, steel & aluminum together can get kinda nasty.
Id drop the $$$ & have something I know works,, look at this. The dimensions are already worked out, proven design,
Theres gonna be a lot of "discovery" time involved in building that first one. This guy has it down & doesnt do one at a time, I know you can do it,
but Ill bet a dollar youll be unhappy that you didnt buy it instead,.
If you can team up & do it in 10 hours, Ill be surprised & if ya do, thats 20 man hours, anything over 10 man hours in the project & youre going backwards, because your time is worth something, youll use materials,
& its possible that when youre done, it wont really be as nice..