Here's another option:
The stringers are premade steel from this outfit
https://www.fast-stairs.com/.
The kit includes two bottom sections of the stringer (pair on the right) and two upper sections (that attach to the landing/top of the staircase).
In between these two pieces on each stringer is an insert that determines the overall length of the stringers as well as the angle of the staircase - in my case, the inserts each had two treads.
These stringer sections are connected together using the connecters at the top of the above photo. Note each has 6 pre-drilled holes that align with the holes on the base and upper pieces. These connectors slide into a channel on the back side of each stringer piece.
Close-ups of the final assemblies.

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The kit does not come with treads. The kit can not include a center stringer; just left and right.
I wanted a 4' wide staircase. The manufacturer said I would need at least 1.5" thick hardwood treads. Neither Home Depot nor Lowes carry that -- both will special order them but there was a significant time delay and even more $ignificant co$t.
Solution: I bought 4x butcher block table tops. They are 48" x 36". Each provided 3x threads made of 1.625" thick maple (they advertise them as 1.75" thick but that must be a rough measure). Actually, each tread (aside from the top one) worked out to 44.25" wide cuz they fit inside the stringers.
Here are a few shots during installation:
- Note the hanging straps can adjust the height the top step; all the other steps are fixed by the design of the stringers
- Note the feet at the bottom of the staircase that rest flat of the floor
- Note a clear photo of the connectors on each stringer

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The treads screw into the step ledges from the bottom
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Done

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There is no provision from the manufacturer for a railing - you'd need to fabricate that. I opted NOT to have a hand rail - if I'm putting a hood / bench seat/ dashboard in my loft, I didn't want to be steering it around a railing.
Final Form

The floor of the loft serve as a hand hold when I am climbing the stairs with something in my hands