[tldr] I'm looking at designing a cage as well, BUT: I'm not a tech inspector for Lemons; this is just my thought process:
I read the manual as describing - at least for the main hoop - [and I presume the front hoop as well] that the vertical member(s) should be just about that - vertical.
P.12 describes "backstays" as required to be "right around 45° off _horizontal_". The diagram indicates an angle between the backstay and the main hoop as also 45°. Since that closes a triangle, that means the vertical member of the main hoop must be "right around perpendicular to the floor".
In each diagram, the front vertical member appears parallel to the rear vertical member; I presume that's deliberate.
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I did not find the section in the manual requiring a "fixed back brace".
I did find the diagrams suggest the "retaining bar/harness bar" (welded to the cage) OR "fixed seat back brace".
I do see the rules strongly recommend a "permanently attached seat back brace", but I don't read that as requiring it to be "fixed". I read that as "a brace permanently attached to the seat back", perhaps with a support bar which can be pinned in several places, which would then allow movable seats [yay!]
The rules indicate:
3.F.1.c Seats Without Seatback Braces. If a seatback brace is not used, a strong, seat-width element such as a shoulder-harness bar must be located within six inches of the seatback to prevent the seat from failing rearward.
3.F.1.d Solid Mounting. All seats, including seats on adjustable tracks, must show minimal looseness and no back-and-forth freeplay.
My conclusion was that we have six inches of adjustment for a seat on tracks with _just_ the welded harness bar; OR, more, but: must have a seatback brace permanently attached to the seatback (but it could be fabricated to be adjustable in tandem with the seat), OR, a fixed seat position.