adjustable square for SHS / box sections

Hi all

Working with
Rectangular Hollow Sections
Square Hollow Sections
box sections
...

The rounded corners make using an adjustable square / sliding square
problematic.

Given number of machine parts I have to make quite accurately based on
SHS's, thinking of a tool specially optimised for marking distances
from the surfaces.
ie. whatever presents as "top", slide along marking down the "sides".
Typically the middle of the "sides".

Accuracy - well using a chalk / soapstone is plenty good enough.
Have to bring magnetic-base drill to the job most of the time, and
with using a shell-cutter / trepanning cutter with a central "pin" to
locate on the centre-pop - accuracy there is less than chalking gives.
So - would hold thing in place and chalk-mark.

I could trim offcuts of angle-iron to make single-dimension tools -
eg. marks 25mm from "top" surface.
Advantage with this or "settable" tool is spin around 180deg and mark
the other "side" and the two heights/levels should match-up.

Hope described this well enough.

Best wishes