Hi David
More or less what I done mate
I got a small piece of 10mm flat bar. cut it to the width that i needed, Rounded the edges with a flap disk.
The end result was a rather nice dolly that I could place inside the round tube when it was partly flattened.
This prevented over flattening & distorton of the tube
I would make you a couple of pieces up but alas....I have no more of the 25mm seamless tube left
Can't remember the length either & cannot get to may chassis to measure it for you.
140-150mm seems to ring a bell though !!??