I use the blue Irwin clamps from HD. They pop open easily and don’t seem to need much pressure to close. I would say they clamp about as tightly as you would want for fabric. I know some people use binder clips, but I’m not looking to smash the fabric, myself, plus binder clips are very hard to open and hold like there’s no tomorrow.
Irwin clamps come in different sizes. All you need is to find one that fits your island top.
hugs, charlotte