I bought a few Quick-Grip clamp/spreaders when they 1st came out.
When they introduced the larger ones I considered them too expensive so altered the 12" ones I had.
I picked up a 20' length of steel of the same size and cut it to different lengths.
The corners needed filing to round them off.
A hole for the screw holding the fixed head on finished it off.
I now have a variety of lengths to choose from.
It only takes a few seconds to move the heads from one steel bar to another.
The longest at present will clamp a metre wide boards.
I have enough spare steel to make a few more.
The 20ft steel bar cost less than a single clamp.