I find these clamps very useful indeed:
And I've just found a quick and dirty way to extend their length:
- Take three clamps.
- Remove the reversible ends of two of them.
- Holding them aligned so that the handled jaws face each other, fit the pin from one bar into the hole on the other, and vice versa.
- Keep them together with the third clamp across the overlap.
The bars lock together side-by-side, making a single long clamp almost double the length of the original ones. It's a bit clumsy using a similar sized clamp just to keep the bars locked together, but anything would do - possibly even a spring clip.
It's not a replacement for a longer bar, but it does get you out of a hole, so to speak. I've got some holding a split chair seat together at the moment whilst it's glueing.