That step kind of confussed me too.

It's for deck and has nothing to do with holding the hatch in place
Thats the way I understood it, after I guled.

Glue the top deck former first, then the rails to the side, top sheeting
and sand it flush to the fuselage.

I glued the rails in place like the instruction and had to cut them off or out.
I found out when the precut sheeting piece wasn't wide enough.
And those formers has a slight angle, so it can't just be drop in.

Think about it, if rails goes on the bottom. It'll hit the cross piece
that has the bind nut on it.

or did i get it totally wrong ?

The dowl on the rear of the canopy and the screw keeps everything in place.

I guess you can glue 1/2" lone piece to prevent from going side to side,
measure the thickness of the fuse and glue the small rails accordingly.
why would it go side ti side when it's locked in place by the dowl and screw ?

I didn't had problems