Re: adjustable square for SHS / box sections

Hello everyone
Your various ideas caused a thought to "gel".
I will try this.
Get a smooth piece of plate a bit wider than the box-section.
Place your "ordinary" adjustable square on it, at the edge.
Make the blade/rule of the adjustable square protrude beneath the plate
lower surface by the amount you want to mark-off - eg. 25mm.
Bring "free" plate's edge to align with "side" surface of box you want
to mark, with the already-set adjustable square on it, and with hand
pressure keeping eveything touching where it should, slide along with
chalk under tip of adjustable-square blade.
Spin plate with adjustable square through 180deg and do same marking
down other side.
Those "heights" on the two "sides" will agree - probably the most
important thing.
Mark distances from the end of the box at which holes are and centre-pop
at the intersections.
Drill.
Done.

Hope this makes sense...

Will see whether this works - and report back, volente dio.