As a professional builder of hollow wooden paddle boards I have a lot of clamps. One of the handiest clamps I use are these spring clamps made from 4" Schedule 20 perimeter drain pipe. These clamps were originally made for building the rails on my wood paddle board kits, but this summer my wife started stealing them for holding down row-covers in her garden. The clamps do not have crushing clamping force but for their weight they are surprisingly strong. There are many things that make these clamps really awesome:
They can clamp at weird angles.
They can be used individually with a very light touch or nested together to increase their clamping force.
They stay put on narrow edges where other clamps eject themselves
They are rust-proof
They weigh next to nothing
Probably the coolest thing is they are made from the waste cutoffs that would otherwise end up in the landfill. Every new construction project throws out enough perimeter drainage pipe cutoffs to fill a Rubbermaid with these clamps. It is because of this upcycling benefit that I feel this secrete should not be limited to just wood surfboard builders.